Switch to NIHIL as results for PRINT, HELP, etc.

When a function doesn't have an "interesting" result, there
are tradeoffs for the choices of what to return.

* VOID has the positive property of making no console
  output, but the negative property of being too widely
  tolerated as an "interesting" way of opting out of a
  parameter.

* Things like NULL and NONE are good for their "disruptive"
  nature, but have the downside of making confusing-looking
  console output which mixes in with the intended output.
  It's a bit ugly, and will make beginners scratch their
  head when using functions like PRINT or HELP.

* NIHIL is the means of vaporizing completely--used by
  functions like COMMENT and ELIDE.  It is slightly unfriendly
  in the sense it cannot be used in assignments or passed as
  an argument to nearly all functions (excluding things like
  RETURN), but may lead to behavior that might seem mysterious
  by preserving the last result in an evaluation chain.

I'd been reticent to make too many invisible functions in the
core, preferring that people write ELIDE PRINT if they really
wanted to erase the return value from PRINT.  But as it became
clear that VOID was truly the best choice for making no
console output, and no console output would be output from
something like `do [comment "hi"]`, the fact that the evaluator
would be based on DO meant that usage of an expression that
was solely an invisible would wind up appearing VOID while
having some ornery properties.

From a marketing perspective, the choice of NIHIL as the
return result from functions like PRINT and HELP simply could
not be avoided.  This buries the hatchet and makes that choice.

src/mezz/base-defs.r

@@ -561,33 +561,36 @@ to-lit-path: func* [return: [quoted!] value [any-value!]] [
 ]
 
 print: func* [
- {Textually output spaced line (evaluating elements if a block)}
+ {Output SPACED text with newline (evaluating elements if BLOCK!)}
 
- return: "NULL if blank input or effectively empty block, else none"
- [<opt> none!]
- line "Line of text or block, blank or [] has NO output, newline allowed"
+ return: "See NIHIL docs for caution on casually making vaporizing routines"
+ [<nihil>]
+ line "Line of text or block, [] has NO output, CHAR! newline allowed"
  [<maybe> char! text! block! quoted!]
 ][
  if char? line [
  if line <> newline [
  fail "PRINT only allows CHAR! of newline (see WRITE-STDOUT)"
  ]
- return write-stdout line
+ write-stdout line
+ return nihil
  ]
 
- if quoted? line [ ; Feature: treats a quote mark as a mold request
+ if quoted? line [ ; Speculative feature: quote mark as a mold request
  line: mold unquote line
  ]
 
- return write-stdout (maybe spaced line) then [
+ write-stdout (maybe spaced line) then [
  write-stdout newline
- ] else [none]
+ ]
+
+ return nihil
 ]
 
 echo: func* [
  {Freeform output of text, with @WORD, @TU.P.LE, and @(GR O UP) as escapes}
 
- return: <void>
+ return: [<nihil>]
  'args "If a BLOCK!, then just that block's contents--else to end of line"
  [any-value! <variadic>]
  <local> line
@@ -614,6 +617,7 @@ echo: func* [
  ]
  ]
  write-stdout newline
+ return nihil
 ]
 
 

src/mezz/mezz-help.r

@@ -81,7 +81,7 @@ browse: function [
 help: function [
  "Prints information about words and values (if no args, general help)."
 
- return: <void>
+ return: [<nihil>]
  'topic [<end> any-value!]
  "WORD! whose value to explain, or other HELP target (try HELP HELP)"
  /doc "Open web browser to related documentation."
@@ -143,7 +143,7 @@ help: function [
  upgrade - check for newer versions
  usage - program cmd line options
  }
- return void
+ return nihil
  ]
 
  ; HELP quotes, but someone might want to use an expression, e.g.
@@ -156,7 +156,7 @@ help: function [
  if match [group! get-word! get-path! get-tuple!] :topic [
  topic: reeval topic else [
  print "NULL is a non-valued state that cannot be put in arrays"
- return void
+ return nihil
  ]
  ]
 
@@ -184,7 +184,7 @@ help: function [
 
  browse join https://r3n.github.io/topics/ as text! topic
  print newline
- return void
+ return nihil
  ]
 
  text! [
@@ -198,7 +198,7 @@ help: function [
  ] else [
  print ["No information on" topic]
  ]
- return void
+ return nihil
  ]
 
  path! word! [
@@ -209,12 +209,12 @@ help: function [
  null? binding of topic
  ] then [
  print [topic "is an unbound WORD!"]
- return void
+ return nihil
  ]
 
  if '~attached~ = binding of topic [
  print [topic "is ~attached~ to a context, but has no variable"]
- return void
+ return nihil
  ]
 
  switch/type value: get/any topic [
@@ -225,7 +225,7 @@ help: function [
  print [topic "is not defined (e.g. has a NONE! value)"]
  ]
  ] then [
- return void
+ return nihil
  ]
  enfixed: (action? :value) and (enfixed? :value)
  ]
@@ -241,7 +241,7 @@ help: function [
  if free? :topic [keep "a *freed*"]
  keep any [mold kind of :topic, "VOID"]
  ]
- return void
+ return nihil
  ]
 
  ; Open the web page for it?
@@ -288,7 +288,7 @@ help: function [
  ] else [
  print [topic {is a datatype}]
  ]
- return void
+ return nihil
  ]
 
  match [action! activation!] :value else [
@@ -312,7 +312,7 @@ help: function [
  ]
  ]
  ]
- return void
+ return nihil
  ]
 
  ; The HELP mechanics for ACTION! are more complex in Ren-C due to the
@@ -400,14 +400,14 @@ help: function [
  print-args/indent-words refinements
  ]
 
- return void
+ return nihil
 ]
 
 
 source: function [
  "Prints the source code for an ACTION! (if available)"
 
- return: <void>
+ return: [<nihil>]
  'arg [word! path! action! tag!]
 ][
  switch/type arg [
@@ -425,7 +425,7 @@ source: function [
  name: arg
  f: get arg else [
  print [name "is not set to a value"]
- return void
+ return nihil
  ]
  ]
  ] else [
@@ -448,7 +448,7 @@ source: function [
  ]
  ]
  ] then [
- return void
+ return nihil
  ]
 
  ; ACTION!
@@ -476,7 +476,7 @@ source: function [
  print "...native code, no source available..."
  ]
 
- return void
+ return nihil
 ]