HDPath: add `Prefix.M` and `Prefix.m` shortcuts
~~2 commits, child of PR #3719 ~~
Making DRAFT for now.
The Prefix.M / Prefix.m addition is controversial and not needed for short-term, so we can shelve that for now.
The deprecated constructor may have to be eliminated entirely due to the addition of full-path and partial-path subclasses.
@schildbach I accidentally pushed a ~~bunch~~ few more commits on this branch/PR. ~~I'll sort it out tomorrow.~~ I sorted it out and submitted them separately in PR #3723
Ok, this is rebased and only has a single commit now. I changed the title as well.
The proposed addition is not very important and is also controversial, so I'll leave this as draft.