Partial merge of _etoa and _ftoa: We now have a single `sprint_fl…

…oating_point()` function, which performs the common work and allocates the temporary buffer necessary storing the reversed digits. It then calls one of two functions - but these are not quote like `ftoa()` and `etoa()` in that they can make stronger assumptions about their input and can skip some of the work.

Also, some renaming.