Use ValueExpression instead of Primary|Operation
source-gen: Add missing newlines in EnumeratedTypeGenerator
RedHatTransformer: use operator instead of hardcoded string

Use the Operation.operator attribute instead of a hardcoded "++"/"--"
string. Who knows, it might change in the future!

(Well, _these_ probably won’t ever change. And I definitely don’t have
any plans to change the text of other tokens in the future… ahem. Oh,
look, over there! A distraction!)
Add BinaryOperation, ExponentiationOperation

and aliases for operator precedence (see #13).

ExponentiationOperation includes a really ugly workaround for a bug that
may or may not be ceylon/ceylon-compiler#1728; I’ll investigate that
some more.
Add tests for BinaryOperation, ExponentiationOperation