Investigate shrinking code size by removing strings from builds

We suspect error strings are a major code size offender, and revamping `wasmtime::Error` to optionally (based on compile-time features) contain just error codes, instead of full strings, could improve code size.

And then for regular Wasmtime CLI builds, we would probably want something like

```
$ wasmtime --explain 1234
Error 1234:

```

Apparently the `defmt` and/or `pw_tokenizer` crates could be helpful here, although I have no experience with either of them.