this also makes the rust.docs-minification option work
as advertised in config.toml
nothing fancy this time, this is intended to be perma-unstable.
it's only really here for the benefit of rustdoc devs.
mitegates https://github.com/rust-lang/rust/issues/135345
* All new functionality is under unstable options
* Adds `--merge=shared|none|finalize` flags
* Adds `--parts-out-dir=<crate specific directory>` for `--merge=none`
to write cross-crate info file for a single crate
* Adds `--include-parts-dir=<previously specified directory>` for
`--merge=finalize` to write cross-crate info files
* update tests/run-make/rustdoc-default-output/rmake.rs golden
