Use generatos in 5.5

[henrikbjorn]

When using 5.5 using iterators is quicker than checking for values and mapping a single value for each tick in the loop.

This also makes it possible to just assume all values are mapped by wrapping the given iterator in a generator.

example went from ~30s to ~15s.

[henrikbjorn]

ref: https://gist.github.com/henrikbjorn/fc09fec9cf4cea4ab085

[henrikbjorn]

To keep BC a MapperIterator have been introduced which wraps a Iterator and a Mapper and it is done transparently in MapReduce::run().