Add PrimaryType (abstract, no RedHat equivalent)

I might add a backend RedHat → Ceylon conversion later, but not before
all the subtypes are done (missing: Tuple-, Optional-, Sequence-,
CallableType).
Add OptionalType

    String?
source-gen: Fix bug with splitType

How the fuck does that error __still__ get me every now and then?
I hate it so much.
source-gen: Fix bug with equals()

The outer 'is ``type`` that' check was missing.
source-gen: Better english grammar (a vs an)
source-gen: Fix unindented line in class body