str: common logic for replace & replacen

[ljedrz]

The logic of str::replace and str::replacen is almost identical - we can use a common helper function. The initial capacity of the resulting string was different (0 vs. 32), but I propose a different approach: use the same capacity as in the original string; this will neither reallocate too often (or not at all if the substitute's length is the same as the substituted bit's), nor will it overallocate for short strings. It will also be an exact fit in case of no pattern hits.