v0.4.5

Various minor improvements.

• Malachite is now no_std, unless the random, test_build, or bin_build features are enabled. Thanks to shekohex for initiating this work!
• As suggested by amamic1803, a twos_complement_limb_count method has been added to Integer.
• As pointed out by amamic1803, the malachite meta-crate did not have a 32_bit_limbs feature to propagate to its sub-crates. Now it does, along with a random feature.
• I wrote a comment auto-formatter, eliminating the time I spend manually formatting comments. The newly-formatted comments account for most of the diff.
• Integer now implements CheckedDiv. Natural now implements PartialOrdAbs for primitive floats. I added some tests for the AbsDiff trait in malachite-base in preparation for implementing it for Natural, Integer, and Rational.
• The random Float generators were producing floats whose exponents were off by 1. That has been fixed.

Most of my work in the last few months has been implementing addition and subtraction for Floats. This is a lot less trivial than it sounds, because I'm translating MPFR's code, which is very optimized and has many special cases for addition and subtraction. Counterintuitively, implementing more advanced functions will take less time. I'm wrapping up the addition and subtraction work, so that will be released soon.