add `patch` command

[gnunicorn]

With substrate / polkadot / ink and alike, we have a few workspace repositories that often want/need a particular un-released version of all crates of a different repository (namely substrate) to build and test against. This is a tedious process that should be automated by adding a new patch command. This would:

Given another source-workspace-manifest, this puts up a [patch.crates-io]-section into the target, adding all crates with the path from the source-workspace. A subsequent run then would use the local checkout rather than the crates.io version.

Similarly we need a command to clean up the patch-section.