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.

cranelift/wasm/src/lib.rs

@@ -65,11 +65,7 @@ pub use crate::func_translator::FuncTranslator;
 pub use crate::module_translator::translate_module;
 pub use crate::state::func_state::FuncTranslationState;
 pub use crate::state::module_state::ModuleTranslationState;
-pub use crate::translation_utils::{
-    get_vmctx_value_label, DataIndex, DefinedFuncIndex, DefinedGlobalIndex, DefinedMemoryIndex,
-    DefinedTableIndex, ElemIndex, FuncIndex, Global, GlobalIndex, GlobalInit, Memory, MemoryIndex,
-    SignatureIndex, Table, TableElementType, TableIndex,
-};
+pub use crate::translation_utils::*;
 pub use cranelift_frontend::FunctionBuilder;
 
 // Convenience reexport of the wasmparser crate that we're linking against,