Switch wasmer FFI boilerplate to new API

Broadly speaking, the new API is very similar to the old one. The only
public facing change is that you can't make a memory by itself anymore.
You need to construct if from a store, which is owned by a module.

The new API no longer passes structs by value, so I was able to delete
the wasmer_wrapper.cc, and the files I had copied from the wasmer repo.

The new API is also a lot more verbose, so it made more sense to write
a script to generate the FFI boilerplate, rather that write it all
manually.

All the migrated tests that were passing before this refactor are still
passing.

Bug: https://github.com/dart-lang/sdk/issues/37882
Change-Id: I52ca54b982a27f1d851c630d3e09833b8810060c
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/164081
Reviewed-by: Siva Annamalai <asiva@google.com>
Commit-Queue: Liam Appelbe <liama@google.com>