Implement the module linking alias section

[alexcrichton]

This commit is intended to do almost everything necessary for processing
the alias section of module linking. Most of this is internal
refactoring, the highlights being:

• Type contents are now stored separately from a wasmtime_env::Module.
  Given that modules can freely alias types and have them used all over
  the place, it seemed best to have one canonical location to type
  storage which everywhere else points to (with indices). A new
  TypeTables structure is produced during compilation which is shared
  amongst all member modules in a wasm blob.

• Instantiation is heavily refactored to account for module linking. The
  main gotcha here is that imports are now listed as "initializers". We
  have a sort of pseudo-bytecode-interpreter which interprets the
  initialization of a module. This is more complicated than just
  matching imports at this point because in the module linking proposal
  the module, alias, import, and instance sections may all be
  interleaved. This means that imports aren't guaranteed to show up at
  the beginning of the address space for modules/instances.

Otherwise most of the changes here largely fell out from these two
design points. Aliases are recorded as initializers in this scheme.
Copying around type information and/or just knowing type information
during compilation is also pretty easy since everything is just a
pointer into a TypeTables and we don't have to actually copy any types
themselves. Lots of various refactorings were necessary to accomodate
these changes.

Tests are hoped to cover a breadth of functionality here, but not
necessarily a depth. There's still one more piece of the module linking
proposal missing which is exporting instances/modules, which will come
in a future PR.

It's also worth nothing that there's one large TODO which isn't
implemented in this change that I plan on opening an issue for.
With module linking when a set of modules comes back from compilation
each modules has all the trampolines for the entire set of modules. This
is quite a lot of duplicate trampolines across module-linking modules.
We'll want to refactor this at some point to instead have only one set
of trampolines per set of module linking modules and have them shared
from there. I figured it was best to separate out this change, however,
since it's purely related to resource usage, and doesn't impact
non-module-linking modules at all.

cc #2094

[alexcrichton]

It's also worth pointing out that this builds on #2447, so the first commit can be ignored here.

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[peterhuene]

Looks great, thanks! Just one comment on filing a follow-up issue for a seemingly important TODO and some nits.