Add pointer to anyCompilationUnitToCeylon to compilationUnitToCeylon
Add pointer to anyCompilationUnitToCeylon to compilationUnitToCeylon

This has tripped up at least two people already (@sadmac7000,
@jvasileff), so it’s clearly necessary. (I agree that
`compilationUnitToCeylon` is a natural candidate for “convert a
compilation unit to ceylon.ast”.)

ceylon/ceylon.ast