perf(serve): Reduce duplicate rebuilds
perf(serve): Reduce duplicate rebuilds

This mostly resolves #356.  vim will cause a `delete` and `create`
notification on save.  The problem is that debouncing is really a
per-application policy rather than something that is universal.  It'd be
ideal for these to be debounced for cobalt but that wouldn't work in
other cases.

cobalt-org/cobalt.rs,cobalt-org/cobalt.rs,cobalt-org/cobalt.rs,cobalt-org/cobalt.rs