Convert Cabal/changelog to markdown [skip ci]

NOTE: this was cherry-picked from 3 commits from `master`
      but sans the 2.4 entries

Notational conventions used:

 - `----` separators to group major version series
 - `#` headings to highlight major version bumps,
 - `##` headings for minor version bumps, and
 - `###` headings for patch-levels

(cherry picked from commit 8bf5a2aa)

Add more backtick markup to Cabal changelog [skip ci]

(cherry picked from commit be676364)

Replace email autolinks by name-labeled hyperlinks [skip ci]

(cherry picked from commit 1d61425a)