* I do not use 1.9.3
* Also keeping them in each file increases mental overhead (true it *can* be autoamted)
* None of the files encodes NON ASCII chars.
* I do not expect it makes any difference, since nobody programmatically
will consume strings generated by mutant under the assumption they are UTF-8 encoded.
* 1.9.3 Users have to deal with the encoding fuckup under ruby anyways.
* Also move index reference / assignment mutators to dedicated file
* Move attribute assignment cases to subclass
* Mutations emitted never had been invalid. Since invalid AST was parsed
to valid source as unparser trusts the AST.
This is a reaction to the to_source rewrite
* Do not use 1.8 specific nodes anymore
* Do not generate AST nodes that would not be generatable from source
