Refactor the TypeTables type (#3971)

* Remove duplicate `TypeTables` type

This was once needed historically but it is no longer needed.

* Make the internals of `TypeTables` private

Instead of reaching internally for the `wasm_signatures` map an `Index`
implementation now exists to indirect accesses through the type of the
index being accessed. For the component model this table of types will
grow a number of other tables and this'll assist in consuming sites not
having to worry so much about which map they're reaching into.

crates/cranelift/src/compiler.rs

@@ -256,7 +256,7 @@ impl wasmtime_environ::Compiler for Compiler {
         let compiled_trampolines = translation
             .exported_signatures
             .iter()
-            .map(|i| self.host_to_wasm_trampoline(&types.wasm_signatures[*i]))
+            .map(|i| self.host_to_wasm_trampoline(&types[*i]))
             .collect::<Result<Vec<_>, _>>()?;
 
         let mut func_starts = Vec::with_capacity(funcs.len());