Removed dead code from Core/Typecheck
Lifted TODO/FIXME to the issue tracker for Core/Typecheck
Lifted a FIXME from Core/ProofState
Lifted FIXME and TODO from Core/CaseTree
Removed dead code and lifedt TODO/FIXME from Core/Execute.
Lifted TODO/FIXME in Core/Unify
Lifted TODO from Core/Tt.
Lifted FIXME from Core/Evaluate.
Lifted FIXME from Core/Constraints.
Merge pull request #1726 from jfdm/fix/lift-fixme-todo

Lifted FIXME and TODOs from Core.