Implement signed fixed point decimal numbers

[ascjones]

See https://solidity.readthedocs.io/en/develop/abi-spec.html#types

Ran across this as part of https://github.com/paritytech/parity/issues/7162. The casper contract is written in vyper and uses the decimal type. In the abi generated by the vyper compiler it is aliased as decimal10.

However there is an ongoing discussion going on to instead use a fixed<M>x<N> type instead (specifically fixed168x10).

PR in eth-abi (python)