v0.4.15

This is a small release. Its purpose is mostly to include this pull request, which specifies the minimum supported Rust version. There is a more substantial release coming in the next weeks, which will contain efficient implementations of Float division and reciprocation.

This release includes

• A MSRV specification, as mentioned above (thanks to John Vandenberg)
• Implementations of AbsDiff for Integer
• Implementations of EqAbs between Rationals and other numeric types
• A small efficiency improvement for the Sum and Product implementations on Rational
• More thorough testing for division-by-zero and exact rounding in malachite-base. This is purely proactive; no bugs have been found