WIP: looks like we will need to cut over to axum after all

Stable dist serving with proper index.html fallback after evaluating
accept header.

Add to changelog:
- [x] async everything in tools.
- [x] add a subcommand to clean cached tools (wasm-bindgen & wasm-opt).
- [x] resolves #198

TODO:
- [ ] ensure the shutdown broadcaster is closed as soon as a signal is
  received, else ^C too early will cause the signal to be missed and
  other parts of the app which have not initialized will not see the
  drop.

Author: thedodd