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.

Fixes #575. Specs added https://github.com/sass/sass-spec/commit/eaad1c6bd63961c3892465dcefd61d94fe89c2f9, https://github.com/sass/sass-spec/pull/110.