avoid allocations

[timotheecour]

/cc @disruptek

• avoid allocations if user didn't ask for it
• allow drop in replacement to system.addFloat
• refs: https://github.com/nim-lang/RFCs/issues/191

[timotheecour]

PTAL

• rebased to resolve conflict from other PR
• having a drop-in replacement for addFloat is valuable, otherwise there'll always be a concern about GC causing some (maybe small comparatively to f2s, but still) performance hit. https://github.com/nim-lang/RFCs/issues/191 should have all the context for such kind of things

[disruptek]

Yeah, that makes sense.