Compiler: simplify DoIt

[privat]

The management of DoIt (executable scripts) is a little hairy and complex.

• Invisible synthetic return statements are injected in parseDoIt and asDoIt, but not in parseExpression.
• asDoit return a RBMethodNode instead of a RBoItMethodNode
• asDoit results have an inconsistent source
• some AST methods are unneeded extension methods

This PR tries to make the behavior less complex:

• original source is not altered (even after asDoit)
• no returns are injected in the AST. The default return behavior is implemented at compile time by the transformator, as it already decides for classic methods with/without a final return.

[jecisc]

Nice!

Only thing I'm not sure about is OCASTTranslator >> visitMethodNode: since I don't know well the compiler, but the code seems good :)

[MarcusDenker]

Nice!