Add FunctionArgument (RedHat’s MethodArgument)

Not to be confused with RedHat’s FunctionArgument, which is ceylon.ast’s
FunctionExpression.

(I’m not using value named arguments in the RedHat AST → ceylon.ast
conversion – see last commits – because it appears they’re up to 7 times
slower due to the anonymous class that they’re implemented with :-( I’ll
probably create a ceylon-compiler issue for that; at least for
“toplevel” invocations, it should be possible to “inline” the anonymous
class body into the surrounding body.)
Add tests for FunctionArgument backend conversion