Add numeric functions and `traits` module that provide numeric traits for generic
mathematcs.

Import numeric traits and functions that were removed from the standard library.
This also implements `AdditiveIterator` and `MultiplicativeIterator` for
`BigUint` and `BigInt`.

closes #28
Add `iter` modules for generic number iteration.

The codes are imported from the standard library.
std::from_str::FromStr -> std::str::FromStr
Merge pull request #30 from gifnksm/master

Add numeric functions and traits that provide numeric traits for generic mathematics.