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/runtime/src/instance.rs

@@ -28,10 +28,10 @@ use thiserror::Error;
 use wasmtime_environ::entity::{packed_option::ReservedValue, BoxedSlice, EntityRef, PrimaryMap};
 use wasmtime_environ::wasm::{
     DataIndex, DefinedFuncIndex, DefinedGlobalIndex, DefinedMemoryIndex, DefinedTableIndex,
-    ElemIndex, FuncIndex, GlobalIndex, GlobalInit, MemoryIndex, SignatureIndex, TableElementType,
-    TableIndex, WasmType,
+    ElemIndex, EntityIndex, FuncIndex, GlobalIndex, GlobalInit, MemoryIndex, SignatureIndex,
+    TableElementType, TableIndex, WasmType,
 };
-use wasmtime_environ::{ir, DataInitializer, EntityIndex, Module, TableElements, VMOffsets};
+use wasmtime_environ::{ir, DataInitializer, Module, TableElements, VMOffsets};
 
 /// A WebAssembly instance.
 ///
@@ -325,6 +325,10 @@ impl Instance {
                 global: self.module.globals[*index],
             }
             .into(),
+
+            // FIXME(#2094)
+            EntityIndex::Instance(_index) => unimplemented!(),
+            EntityIndex::Module(_index) => unimplemented!(),
         }
     }