Implement StringMethod slice_replace
The `StringMethod` `slice_replace` currently raises `NotImplementedError`. Couldn't find an issue for it, so opening this. PR with implementation in the works.

Could use thoughts on how to handle slices that return an empty string on some subset of strings - should the replacement string appear or not? This could happen when the input string is empty, the stop index is less than or equal to the start index (separate cases?), or both start and stop are greater than the length of the string.
