Module registry: investigate data-structure optimizations

In #12051 we added a new level of data structures to the per-store module registry due to the distinction between `EngineCode` and `StoreCode`. This leads to a small but consistent observed increase in instantiation time (~3% or 40ns in tests with a store of one small module).

In theory we could avoid the cost of materializing a separate `StoreCode` by making `ModuleWithCode` carry a view on *some* text/range, and building it to use `EngineCode`'s text slice directly in the non-debug case. The commit [here](https://github.com/cfallin/wasmtime/commit/73b0bbe8d5101e5849a22404f2809895f0ee7495) did enough of this to avoid a lookup in the engine-to-store-code map on the funcref init path, but we could carry it a bit further and avoid creating a `StoreCode` at all at instantiation.

There may be other optimizations we could do as well, after profiling.