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/lib.rs

@@ -37,10 +37,7 @@ pub use crate::address_map::*;
 pub use crate::builtin::*;
 pub use crate::compilation::*;
 pub use crate::data_structures::*;
-// pub use crate::func_environ::BuiltinFunctionIndex;
-pub use crate::module::{
-    EntityIndex, MemoryPlan, MemoryStyle, Module, TableElements, TablePlan, TableStyle,
-};
+pub use crate::module::*;
 pub use crate::module_environ::*;
 pub use crate::tunables::Tunables;
 pub use crate::vmoffsets::{TargetSharedSignatureIndex, VMOffsets, INTERRUPTED};