Better handling of non-ascii filepaths

[jennybc]

Closes #394, closes #402, closes #403 (previous explorations)

There's a bit more going on here than we originally thought and the solution isn't quite as simple as "UTF-8 everywhere" (but it's close).

First, the file from the original issue (tidyverse/readr#1345) has a path containing non-ascii characters, but what's easier to miss is that the file also lacks a trailing newline. The failure cascade looks like this:

has_trailing_newline() implicitly expects that the path is in the native encoding and, if it's not, can potentially fail to find an existing file. When that happens, it unconditionally reports TRUE (yes, there is a trailing new line), which is wrong in this case. A file that lacks a trailing newline should be routed through the connection logic, but, in this case is read as a "regular file", which fails and results in an empty tibble.

So yes the problem is the file path encoding, but it first raises its head via has_trailing_newline(), not the main file-reading step.

Second, by exploring various solutions, I discovered that blindly applying enc2utf8() is too simple, since vroom tests itself on linux in a non-UTF-8 locale. Hence we do enc2utf8() on Windows and enc2native() otherwise. This effort did reveal a pre-existing file path problem on this build that I ultimately decided to not fix, in the "file ends with a newline" case. I think this OS-dependent approach may be currently irrelevant in vroom, because of the mio problem I show below, but might be worth it in other settings.

For normal files, on linux, in a non-UTF-8 locale, with a multi-byte file path, we get an error from mio itself, i.e. the memory mapping fails:

https:/tidyverse/vroom/runs/6315284023?check_suite_focus=true#step:7:182

> test_check("vroom")
mapping error: No such file or directory
[ FAIL 2 | WARN 0 | SKIP 2 | PASS 1065 ]

I am content to skip() here and re-consider if an actual user ever needs to do this. There is also evidence that vroom's writing functions are not entirely prepared for this scenario, but the same posture applies.

[DavisVaughan]

Such a hard fought battle