constants, mapper: type bugfixes

[woodruffw]

My type table state machine thingy is a little bit wrong, since it doesn't correctly handle forward-referenced types (e.g. if a pointer type has struct foo as its pointee).

Needs fixing.

[woodruffw]

This is going to be a little annoying with the current implementation, since we fully "unroll" each type as we encounter it. As a result, forward references are prematurely unrolled and we don't have a chance to fix them up.

[woodruffw]

Braindumping so that I have a better starting point for this later. There are a few annoying aspects of LLVM's type table layout:

• Types are uniquely identified by their index in the type table. Types can have subtypes and those subtypes might not be fully declared yet, so LLVM's own implementation assumes that an index that doesn't already contain a type is a forward-declared structure and puts a default struct at that index. These forward indexes can in turn be "sparse", since a type might further reference multiple forward types before each is visited.
• In LLVM, the equality of types is defined by their pointer identity: (T1 == T2) <=> (&T1 == &T2). This works because types are uniqued on a global/per-module basis, and are really just references to an internal table where all types are stored. Duplicating this in Rust is a massive headache, since it implicitly depends on mutability while other objects hold immutable references: the type table is visited incrementally to fill in e.g. forward types, and every previous type that contains subtypes gets updated "for free" thanks to the data behind their common pointer being updated.