Enable dynamic QAST Tree building

[KennethTrecy]

By using function, the developer can plug different types of QAST node in the AST. For example, upon invoking the function, it may return QAST::Op.new(:op<callmethod>) or QAST::Var.new(:scope<attribute>), depending on the terms that was passed.

[AlexDaniel]

Hello! I invited you to the perl6 org in case you have more fixes :) As for this one, I think we'd need to wait for somebody else to review.