v0.9.1

This release advances Float development.

• AGM (the arithmetic-geometric mean) is now supported. It might seem odd to implement this obscure function before many elementary functions, but it turns out that the AGM is useful in computing logarithms, which will be the subject of the next release. Malachite also provides functionality for taking the AGM of two Rationals and returning a Float result.
• Malachite is now able to compute pi to arbitrary precision. (If you want to benchmark this, beware that the current Float-to-decimal-string conversion is very slow; this will be fixed within the next few months.) Malachite also provides all the pi-related constants that std provides here.
• Malachite can now also compute Gauss's constant and the lemniscate constant to arbitrary precision.
• Rationals now have implementations for the Rem trait (and Mod and CeilingMod, just like Integers). We define x % y to be x - y * sign(x * y) * floor(|x/y|), though the implementaton is slightly faster than the naive one.