Radically simply the map parser logic

This PR radically simplifies the maps parser logic. I've removed the prelexer hack I added to get 3.0 out the door with useful maps. The prelexer hack imposed limitations on what could be in a map key subsequently making #555, #574 impossible. It also changed the parser token resolution flow which meant https://github.com/sass/libsass/issues/575 wasn't possible.

Since this new improved approach restores the existing parser flow #575 is automagically fixed! :party: :boom: