uint: Implement modulo operations for special moduli

[haslersn]

For a project of mine where the modulus can be chosen to be close to UInt::MAX, I created optimized implementations of modular operations. I thought maybe others can benefit from them, too, so I ported my implementation to the crypto-bigint crate.

This commit implements modulo operations (neg, add, sub, mul) for special moduli that are so close to MAX that the difference to overflow fits in a single Limb. For such moduli, these new implementations are much faster than the existing generic modulus implementations. (For mul there's no comparison since there's no corresponding generic modulus implementation, yet.)

For U256, I benchmarked the generic against the specialized implementations using criterion-rs on Intel Core i7-8565U @ 1.80GHz and obtained the following average times. Note that I used a const modulus known at compile-time, which enables some compiler optimizations after inlining. With a modulus known only at runtime, times might differ.

	U256::add_mod	U256::sub_mod	U256::mul_mod
generic (after #109 got merged)	10.857 ns	9.6262 ns	not implemented
_special	3.8276 ns	4.1339 ns	20.188 ns

[tarcieri]

This looks reasonable enough at first glance. Give me a little more time to more thoroughly review it.

[haslersn]

I had an error in my benchmarks, namely I forgot to pass the inputs through black_box. I corrected the original post with updated benchmark results.

[tarcieri]

Thank you!