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/wasmtime/src/trampoline/func.rs

@@ -10,7 +10,7 @@ use std::mem;
 use std::panic::{self, AssertUnwindSafe};
 use wasmtime_environ::entity::PrimaryMap;
 use wasmtime_environ::isa::TargetIsa;
-use wasmtime_environ::{ir, CompiledFunction, EntityIndex, Module};
+use wasmtime_environ::{ir, wasm, CompiledFunction, Module};
 use wasmtime_jit::trampoline::ir::{
     ExternalName, Function, InstBuilder, MemFlags, StackSlotData, StackSlotKind,
 };
@@ -227,7 +227,7 @@ pub fn create_handle_with_function(
     let func_id = module.functions.push(sig_id);
     module
         .exports
-        .insert(String::new(), EntityIndex::Function(func_id));
+        .insert(String::new(), wasm::EntityIndex::Function(func_id));
     let trampoline = make_trampoline(isa.as_ref(), &mut code_memory, &mut fn_builder_ctx, &sig);
     finished_functions.push(trampoline);
 
@@ -274,7 +274,7 @@ pub unsafe fn create_handle_with_raw_function(
     let func_id = module.functions.push(sig_id);
     module
         .exports
-        .insert(String::new(), EntityIndex::Function(func_id));
+        .insert(String::new(), wasm::EntityIndex::Function(func_id));
     finished_functions.push(func);
     store.signatures().borrow_mut().register(wft, trampoline);