Propagate module-linking types to wasmtime (#2115)

This commit adds lots of plumbing to get the type section from the
module linking proposal plumbed all the way through to the `wasmtime`
crate and the `wasmtime-c-api` crate. This isn't all that useful right
now because Wasmtime doesn't support imported/exported
modules/instances, but this is all necessary groundwork to getting that
exported at some point. I've added some light tests but I suspect the
bulk of the testing will come in a future commit.

One major change in this commit is that `SignatureIndex` no longer
follows type type index space in a wasm module. Instead a new
`TypeIndex` type is used to track that. Function signatures, still
indexed by `SignatureIndex`, are then packed together tightly.

cranelift/wasm/src/state/func_state.rs

@@ -7,7 +7,7 @@
 //! value and control stacks during the translation of a single function.
 
 use crate::environ::{FuncEnvironment, GlobalVariable, WasmResult};
-use crate::translation_utils::{FuncIndex, GlobalIndex, MemoryIndex, SignatureIndex, TableIndex};
+use crate::translation_utils::{FuncIndex, GlobalIndex, MemoryIndex, TableIndex, TypeIndex};
 use crate::{HashMap, Occupied, Vacant};
 use cranelift_codegen::ir::{self, Block, Inst, Value};
 use std::vec::Vec;
@@ -236,7 +236,7 @@ pub struct FuncTranslationState {
     // Map of indirect call signatures that have been created by
     // `FuncEnvironment::make_indirect_sig()`.
     // Stores both the signature reference and the number of WebAssembly arguments
-    signatures: HashMap<SignatureIndex, (ir::SigRef, usize)>,
+    signatures: HashMap<TypeIndex, (ir::SigRef, usize)>,
 
     // Imported and local functions that have been created by
     // `FuncEnvironment::make_direct_func()`.
@@ -498,7 +498,7 @@ impl FuncTranslationState {
         index: u32,
         environ: &mut FE,
     ) -> WasmResult<(ir::SigRef, usize)> {
-        let index = SignatureIndex::from_u32(index);
+        let index = TypeIndex::from_u32(index);
         match self.signatures.entry(index) {
             Occupied(entry) => Ok(*entry.get()),
             Vacant(entry) => {