Top-level contracts - Githubissues

This PR adds a lot of generic infrastructure relating to contracts.

Namely:

improvements to Makefile and scripts
Added an execution test suite
Added code for contract definitions, including functions, constructors, and state variables
Added extremely minimal code for expressions (needed for testing): address casts and integer literals
Added support for contract creation transactions
Removed virtual keyword from test suite because we don't support inheritance.

Pi-Squared-Inc / solidity-demo-semantics