The ReduceInto benchmark performs three tasks using both `reduce(_:_)` and `reduce(into:_:)` so that their performance can be compared:
1. Summing an array, reducing to `Int`
2. Filtering an array, reducing to `[Int]`
3. Counting letter frequencies, reducing to `[Character: Int]`
Implement and document `reduce(into:_:)`, with a few notes:
- The `initial` parameter was renamed `initialResult` to match the first parameter in `reduce(_:_:)`.
- The unnamed `combining` parameter was renamed `updateAccumulatingResult` to try and resemble the naming of the closure parameter in `reduce(_:_:)`.
- The closure throws and `reduce(into:_)` re-throws.
- This documentation mentions that `reduce(into:_)` is preferred over `reduce(_:_:)` when the result is a copy-on-write type and an example where the result is a dictionary.
Add benchmarks for reduce with accumulation into a scalar, an array, and a dictionary.
Update expected error message in closures test (since there are now two `reduce` methods, the diagnostic is different).
