Refine cider-locate-def

It now goes to the start of a matched definition.
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