Preserve $OUT_DIR across rebuilds

Preserve $OUT_DIR across rebuilds

The original choice for completely destroying $OUT_DIR was to focus on
repeatable builds to ensure that stale items in $OUT_DIR don't affect later
compilations. This commit moves this responsibility from cargo to the build
command itself.

Build commands are now responsible for cleaning out old artifacts and ensuring
that when invoked the state of $OUT_DIR is as it would be if it started with an
empty $OUT_DIR. This will allow for very long build commands based on `make` to
have a much faster incremental build time as they won't have to rebuild
everything when updated.

This can cause non-repeatable builds with build commands if stale artifacts are
never removed (but still used in the rust compilation step), but this will now
be considered a bug in the build command rather than for cargo itself.

Closes #382