Reduce Allocations in `YieldCurve` - Add Iteration Wrapper to get rate and time?

[alecloudenback]

E.g.:

WithTime(ScenarioGenerator(...))

Would return a vector of tuples with rate and time. This would make, e.g. __fwd_times able to not allocate so much.

Composing iterators alternative: Transducers doesn't retain/compute the collected size in advance, so that didn't work. Try just base iterators?

[alecloudenback]

see #37 for some progress