Support type-tagging declarations like runtimes

[pdarragh]

An older version of write-byte explicitly handled the shifting of the integer argument and the returning of the void value in the assembly, but our modern implementation has shifted these responsibilities to the C runtime. This requires our runtime to be able to return bitwise representations of Racket values.

A new system should be implemented to handle the transformation of Racket values to bitwise representations and back on a per-runtime level. This will also be used by #15 to handle rendering; perhaps there are two stages to the tagging? I'll have to think about how to implement this in a way that doesn't feel terrible.

Unfortunately, work on #16 is halted in the meantime, since the Loot tests added in e4dfa46 can't possibly pass until the write-byte issue is resolved.