Rename and clarify -mapPrevious…, rename all combine: parameters to reduce:

To hopefully clarify its purpose, `-mapPreviousWithStart:combine:` is now `-combinePreviousWithStart:reduce:`, and — along with `-scanWithStart:` — now comes with examples and contrasting documentation.

I also went ahead and renamed all other `combine:` method parameters to `reduce:`, since that's actually the operation we're talking about. The behavior isn't at all like `+combineLatest:`, but it is very similar to `-reduceEach:`.

Fixes #525.