Add tests for the interface types output of wasm-bindgen

[alexcrichton]

This commit expands the test suite with assertions about the output of
the interface types pass in wasm-bindgen. The goal here is to actually
assert that we produce the right output and have a suite of reference
files to show how the interface types output is changing over time.

The reference test suite added in the previous PR has been updated to
work for interface types as well, generating *.wit file assertions
which are printed via the wit-printer crate on crates.io.

Along the way a number of bugs were fixed with the interface types
output, such as:

• Non-determinism in output caused by iteration of a HashMap

• Avoiding JS generation entirely in interface types mode, ensuring that
  we don't export extraneous intrinsics that aren't otherwise needed.

• Fixing location of the stack pointer for modules where it's GC'd out.
  It's now rooted in the aux section of wasm-bindgen so it's available
  to later passes, like the multi-value pass.

• Interface types emission now works in debug mode, meaning the
  --release flag is no longer required. This previously did not work
  because the __wbindgen_throw intrinsic was required in debug mode.
  This comes about because of the malloc_failure and internal_error
  functions in the anyref pass. The purpose of these functions is to
  signal fatal runtime errors, if any, in a way that's usable to the
  user. For wasm interface types though we can replace calls to these
  functions with unreachable to avoid needing to import the
  intrinsic. This has the accidental side effect of making
  wasm_bindgen::throw_str "just work" with wasm interface types by
  aborting the program, but that's not actually entirely intended. It's
  hoped that a split of a wasm-bindgen-core crate would solve this
  issue for the future.