Rewritten Var/values implementation based around flexbuffers

[rdaum]

This basically rewrites most of the values layer to be based around values and collections held in flexbuffers. (Flexbuffers are part of the flatbuffers project, and are a kind of zero copy dynamic flatbuffer.)

Part of a bigger effort to do the following:

• Rip out custom serialization formats and replace with a combination of flatbuffers and flexbuffers
• Same for all uses of bincode2
• Clean up the Var interface more broadly

Along with this change came major cleanups generally to the interface in the values layer:

• Make all index-based operations take an argument stating whether they should operate on one-indexed (like MOO) or zero-indexed (like most languages). This cleans up the ad-hoc way the conversion was happening before, and makes it easier to use languages other than MOO, but using the same values layer.
• Move printing of the literal representation of a value out into the compiler crate, making it clear that the literal printing is of a MOO value, and leaving room for other languages to have other representations, or to add different pretty printing / decompilation options.
• Makes the type system interface more regular in general, providing traits to identify operations for scalar/associative/sequence classes of type.

To be done still:

• Rip out use of bincode2 and custom serialization code and replace with schema'd flatbuffer entities.
• There are definitely lots of excessive copies still happening, and there's likely much more clever things that can be happening inside Var, including switching between a 'building' and 'reading' mode such that a given value doesn't obtain its flexbuffered pickled form until first written to the database. Or something.