Simplify a bit of code
Use `cider-from-nrepl-filename-function` to translate nrepl filenames

Add a default implementation on cygwin systems utilizing "cygpath" to
translate native filenames to cygwin filenames.
Use `cider-to-nrepl-filename-function` to translate Emacs filenames

Add a default implementation on cygwin systems utilizing "cygpath" to
translate cygwin filenames to native filenames.
Remove redundant var: Refs #735
Add a changelog entry for Cygwin support. Refs #735
Add Cygwin test