Bug fix: normalize "\\" == "\\"
Bug fix: normalise "//server/test" == "\\\\server\\test"

When drive represents a network share, slashes were not being normalised
properly.
Refactor normalise: simplify propSep

Only when a path on Windows starts with a leading forward slash does propSep
need to do something, all others path separators are properly taken care of by
joinPath.

Add 2 tests to show that splitDirectories can handle superfluous
pathSeparators, and a test to show that (this new version of) propSep is
indeed needed to normalise relative-to-root paths on Windows.
Merge branch 'normalise' of https://github.com/thomie/filepath into thomie-normalise

Conflicts:
	System/FilePath/Internal.hs
	changelog.md
Merge branch 'thomie-normalise'