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adopt `wit-dylib` (#171)

* adopt `wit-dylib`

This removes the built-in Wasm code generator in favor of `wit-dylib`, which is
designed to be reused by interpreters for various languages.  It avoids
redundant effort and leaves us with less code to maintain overall.

This isn't particularly optimized, yet.  Given how `wit-dylib[-ffi]` works, we
have to acquire and release the CPython GIL a lot more frequently (e.g. for
every parameter and result of every import and export call).  I haven't measured
the overhead in practice, so I don't know if it's significant; if it is, we can
make changes to `wit-dylib` to reduce the overhead.  Also, `wit-dylib` currently
embeds a data section with a lot of metadata that we don't use, so we could slim
that down if needed.

Otherwise, this should not affect the developer experience in any noticeable
way.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

* run `cargo fmt`

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

* update README.md files to reference `wasmtime` release that actually exists

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

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Signed-off-by: Joel Dice <joel.dice@fermyon.com>