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/lib.rs

@@ -226,7 +226,7 @@ fn func_signature(
         _ => wasmtime_call_conv(isa),
     };
     let mut sig = blank_sig(isa, call_conv);
-    push_types(isa, &mut sig, &types.wasm_signatures[func.signature]);
+    push_types(isa, &mut sig, &types[func.signature]);
     return sig;
 }