Boxed uintlike coverage

[xuganyu96]

This is an attempt to implement for BoxedUint the missing arithmetic needed for crypto-primes (see entropyxyz/crypto-primes#37 (comment)).

[tarcieri]

You can cut down on a number of allocations here:

Suggested change

	let (half, is_odd) = a.shr1_with_carry();
	let half_modulus = modulus.shr1();
	let if_even = half.clone();
	let if_odd = half
	.wrapping_add(&half_modulus)
	.wrapping_add(&BoxedUint::one_with_precision(a.bits_precision()));
	BoxedUint::conditional_select(&if_even, &if_odd, is_odd)
	let (mut ret, is_odd) = a.shr1_with_carry();
	let half_modulus = modulus.shr1();
	let if_odd = ret
	.wrapping_add(&half_modulus)
	.wrapping_add(&BoxedUint::one());
	ret.conditional_assign(&if_odd, is_odd);
	ret

[tarcieri]

Most of the downstream crates which use this crate don't need the alloc feature. It's really just for rsa/dsa.