Add rem_floor and rem_ceil

[orlp]

This adds the rem_floor (for all signed and unsigned integers) and rem_ceil (for all signed integers) methods. I found these to be missing from the int_roundings feature. I don't think we should ship div functions without a corresponding rem.

The exception is that there is no rem_ceil for unsigned integers, as it would always return a negative result when non-zero.

r? @jhpratt

[jhpratt]

Nominating for team discussion.