Add VoidModifier
Add tests for VoidModifier backend conversion
CeylonExpressionTransformer: refactor + simplify

transformWithIndent() can now also receive and correctly transform
sequentials of nodes. With this, all the other transform() functions can
be written in abbreviation style, and the source-gen’d version is often
good enough (occasionally, we still want to refine it with an 'that.attr
exists then ... else ...').
Add CallableParameter (RedHat’s FunctionalParameterDeclaration)
Add tests for CallableParameter backend conversion
CeylonExpressionTransformer: small improvement

to transformArgumentList