Remove wee_alloc from WASM builds

Turns out that wee_alloc is deprecated (or at least unsupported, per #33), and so it's better if we don't offer it as an option for WASM builds.
Apologies to the many, many people who no doubt have started using the WASM builds with wee_alloc already. 😁
Thanks to Ben Heidemann (@bcheidemann) for the PR (#37).

This release is also an attempt to provide something for pre-commit to grab a hold of, per #35.