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/environ/src/data_structures.rs

@@ -24,9 +24,5 @@ pub mod entity {
 }
 
 pub mod wasm {
-    pub use cranelift_wasm::{
-        get_vmctx_value_label, DataIndex, DefinedFuncIndex, DefinedGlobalIndex, DefinedMemoryIndex,
-        DefinedTableIndex, ElemIndex, FuncIndex, Global, GlobalIndex, GlobalInit, Memory,
-        MemoryIndex, SignatureIndex, Table, TableElementType, TableIndex, WasmFuncType, WasmType,
-    };
+    pub use cranelift_wasm::*;
 }