Turn the GF compiler into a library. Main program is now in src/programs/gf.hs

The module src/compiler/GF.hs now serves as a prelimiary compiler API. It just
exports a selection of functions and types from the compiler.

Haddock documentation can be generated with

	cabal haddock --hyperlink-source

Also bumbed the version number to 3.6.10.

src/programs/gf.hs

@@ -0,0 +1,3 @@
+import qualified GF
+
+main = GF.main