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.

crates/fuzzing/src/oracles/dummy.rs

@@ -17,6 +17,10 @@ pub fn dummy_imports<'module>(
                 ExternType::Global(global_ty) => Extern::Global(dummy_global(&store, global_ty)?),
                 ExternType::Table(table_ty) => Extern::Table(dummy_table(&store, table_ty)?),
                 ExternType::Memory(mem_ty) => Extern::Memory(dummy_memory(&store, mem_ty)),
+
+                // FIXME(#2094)
+                ExternType::Instance(_) => unimplemented!(),
+                ExternType::Module(_) => unimplemented!(),
             })
         })
         .collect()