Improve accuracy of various functions in edge cases

[mb64]

Fixes #2.

Some of the approximations yielded crazy answers for inputs that they weren't designed to deal with, such as ln giving 10^40 for inputs near 0.3 (the real answer is about -1.2).

This fixes it simply, by adding if statements and transforming it to numbers in the reasonable domain of the approximations. For ln, for example, it computes the log of numbers less than 1 by doing -ln(1/x).

Not all the functions are modified; just the ones I needed to fix edge cases for to use.

Here's my reasoning behind what I changed:

• I graphed all the things I was using, and modified the ones that looked incorrect to be better
• Using these modifications made the test suite for the palette crate pass when it didn't before
• The mish test suite results have not changed