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/environ/src/module_environ.rs

@@ -509,7 +509,7 @@ impl<'data> ModuleEnvironment<'data> {
 
                 if self.tunables.generate_native_debuginfo {
                     let sig_index = self.result.module.functions[func_index].signature;
-                    let sig = &self.types.wasm_signatures[sig_index];
+                    let sig = &self.types[sig_index];
                     let mut locals = Vec::new();
                     for pair in body.get_locals_reader()? {
                         locals.push(pair?);