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/jit/src/lib.rs

@@ -30,7 +30,7 @@ mod unwind;
 pub use crate::code_memory::CodeMemory;
 pub use crate::instantiate::{
     finish_compile, mmap_vec_from_obj, subslice_range, CompiledModule, CompiledModuleInfo,
-    SetupError, SymbolizeContext, TypeTables,
+    SetupError, SymbolizeContext,
 };
 pub use demangling::*;
 pub use profiling::*;