swift234

SE-0113 + residual SE-0067 work (#3443)

Implemented SE-0113 + residual SE-0067 operations.

- adds `rounded` and `round` to `FloatingPoint`, from SE-0113.
- adds `remainder`, `squareRoot`, and `addingProduct`, from SE-0067.
- adds basic test coverage for all of the above.
- provides a default implementation of `nextDown` on `FloatingPoint`.