This fixes an unfortunate series of events which renders most historical
builds of `wasm-tools` un-buildable from crates.io without `--locked`.
The basic problem here is of this shape:

* The `wasm-tools` CLI requires the `wasm-module` feature to be enabled
  in `wast` but doesn't explicitly enable it.
* The `wasm-tools` crate depends on `wat`, which by default enables this
  feature. This is why everything "works" today.
* Once published the `wat` crate has semver-compatible releases, meaning
  that a historical `wasm-tools` may pull in a newer `wat`.
* If a historical `wasm-tools` pulls in a newer `wat` then it'll
  activate the `wasm-module` dependency in a different version of
  `wast`, meaning the CLI doesn't have the feature enabled in its
  dependency of `wast`.

I'm not sure how it would be possible to detect this on CI and fix this
or otherwise have a check against it. Thankfully using `--locked` at
least for historical builds works (but that shouldn't be required).