Create SafeMath32.sol

[wbt]

This PR creates a library that serves the same functions as SafeMath, but for uint32s instead of uint256s. Now, those of us doing math with uint32s can do so safely!
Only variable types, the contract name, and the description on line 5 have changed from SafeMath.sol.

[wbt]

The new tests are the same as the tests being applied to SafeMath. However, the validation tests for the last commit finished before the tests for the one before it. Checking on the Coveralls server UI it looks like the coverage check wasn't run after the tests were added:

Apologies for not renaming the branch as descriptively as is conventional.

[frangio]

Hi @wbt! Thanks for contributing. Unfortunately we've decided in the past to only support 256 bit arithmetic. Since that is the native word size in the EVM and everything else is emulated by Solidity, we don't see any situation where 32 bit arithmetic should be used.

If you have an example situation where you think 32 bit arithmetic is justified, please let us know!

[wbt]

I use 32-bit unsigned integers quite a bit because they are approximately a quarter of the storage cost when packed in, while still being sufficiently large for practical application purposes. Storage is costly in the EVM!

[frangio]

Do you have some sample code to demonstrate the gas savings? Let's continue the discussion in #1625.