rustdoc rfc#3662 changes under unstable flags * 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 * `tests/rustdoc/` tests for the new flags