Support stateMutability

[axic]

For close to a year now Solidity also outputs the stateMutability field for each function. This can have the following values:

• pure
• view
• nonpayable
• payable

For backwards compatibility also the field constant (set to true on stateMutability pure or view) and payable (set to true on stateMutability payable`) is present.

We'd like to phase out the constant and payable fields. For more see https://solidity.readthedocs.io/en/develop/abi-spec.html#json

It seems that abiParser.ts doesn't support it: https://github.com/Neufund/TypeChain/blob/master/lib/abiParser.ts#L47

[krzkaczor]

@axic thanks for your comment. I actually really like this new approach. I will work on improving it when i ll find some time.