Faster compile and smaller build-dir can be at odds with each other.

That said, this release includes some incremental improvements as well as the unused-dependency lint to help reduce wasted work within the ecosystem.

Some changes that have been or are about to be merged for upcoming releases

- We will be reducing the debug info generated for the `dev` profile which reduces build-dir size and speeds up builds. We've split off a `debug` profile for when that is needed.

- Cargo will be deleting the directory rustc writes before each invocation, avoiding the leaking of debug files when split-debug is enabled (default on Mac)

- rlibs for workspace members will no longer be copied from build-dir to target-dir which takes up extra space on systems without hardlinking or where we skip it (like Mac)

- redundant information in rlibs and rmeta's is going to be de-duplicated

Examples of further out changes

- This release includes the foundational work to add a build-dir GC

- Cross-workspace de-duplicating of build-dirs

- Cross-workspace build-dir cache

- Early cut off