In the past, the wasmtime-runtime crate couldn't directly call
`get_wasm_trap` because the registry of loaded modules was in the
wasmtime crate, so it called through a global function pointer
registered with `init_traps` instead.

Since the two crates were merged in bytecodealliance#8501, we no longer need this
indirection.

While I'm here, I've also split the former `get_wasm_trap` function into
two parts: `lookup_code` finds a loaded module that had been previously
registered with `register_code`, and the `lookup_trap_code` step is now
done by a helper on `CodeMemory`. This makes the module registry more
broadly useful.

I also simplified the code lookup step in two ways:
- I removed a redundant check from the code lookup. `BTreeMap::range`
  will only return entries where `end >= pc`, so the `end < pc`
  condition is always false.
- I used checked_sub instead of writing both the comparison and
  subtraction explicitly.