refactorings in preparation for ropes elimination of the C code gener…

[Araq]

…ator; mostly the usual ': Rope' -> 'result: var Rope' rewrite

The goal is to do the refactorings all within the ropes based code generator and then finally replace the ropes by a seq[Token] approach. It is hoped that this saves about 100MB of RAM for boostrapping and consequently for all Nim projects of medium size. Usually memory savings translate also into runtime performance improvements but it's too early to tell.