Value and object representation overhaul

[graydon]

This is a bit more than a sketch of a subset of things we've discussed doing in #679:

• Adapts to XDR changes in Value representation overhaul rs-stellar-xdr#234 (which are in turn reflections of Value representation overhaul stellar-xdr#70).
• In particular: changes the RawVal tagging scheme:
  • Removes two-level tagging in favour of single-level.
  • Expands the tag to a full byte (well, really 7 bits / 128 values, but we round up to 8 for taste / future-proofing).
  • Therefore sadly reduces SmallSymbol to 9 chars; but we have tag-room for a big symbol type that's an object!
  • Adds a full-sized big symbol object type (SymbolObject) and a union between the two (Symbol). Unfortuantely now all the code that used to think Symbol didn't need an Env to work with .. needs an Env.
• Adds a dependency on ethnum, a crate supporting {i,u}256 numbers.
• Adds small-value cases for all number types not just u64.
• Recreates something like ScObject but make it a marker type ScValObject that marks the subset of ScVals that need to be objects (this is a dynamic judgment based on the size of the values in some cases).
• Switches the UDT code to bulk-initialization and bulk-matching (adding host functions to support)
• Adds tag-specific wrappers for every tag case, and tightens all host function signatures to use them (eg. map methods take and return MapObject now, not just Object)

At this point env-common, guest and host are compiling. Tests are about passing and the corresponding SDK change is done too: stellar/rs-soroban-sdk#879

[leighmcculloch]

👏🏻 I really like the design with the small types in the RawVal and how that doesn’t leak into the ScVal anymore.