Implement Solidity Coder

[ricmoo]

The solidity coder provided uses libraries we otherwise already have similar libraries other places (such as sha3 and bignumber) which makes it take up as much space as the entire rest of this library.

The solidity library is also LGPL, which is one of only two components of ethers-wallet which are not MIT licensed. So, it is a fairly small amount of work to move the entire project to a completely MIT licensed project.

Make sure to test against the test cases here:
https://github.com/ethereum/solidity.js/tree/master/test