Smallbigint

A wrapper around num_bigint::BigUint and num_bigint::BigInt that stays out of the heap for small values.

In the current implementation, we go to the heap for anything that doesn't fit in 32 bits.

This crate is already has a lot of relevant methods, but it is not really complete yet. Patches are welcome!

To do, and important:

• Switch the "big integer" enum variant to a Box, as BigInt and BigUint are actually quite sizeable on the stack.
• Implement std::fmt::{Binary, LowerHex, Octal, UpperHex} (easy?)
• Implement num_bigint::{ToBigInt, ToBigUint}
• Unit tests. Currently there are none, although the code is sufficiently simple that there is almost no place where bugs could hide.
• Continuous integration.
• Make this compile against rust stable.

Other traits and methods still to be implemented:

• Bit operations
• num_traits::ops::checked traits
• num_integer::Integer
• num_traits::Num (easy?)
• num_traits::One (easy?)
• std::iter::Product
• num_integer::Roots
• num_traits::Signed
• std::iter::Sum
• num_traits::Unsigned
• Other methods implemented directly on BigInt, BigUint

I probably also want conversions from/to 16-bit and 8-bit types.

Not done and seems hard:

• num_traits::pow::Pow