Remove C backend infrastructure

Issue #384
Remove codegen and icode infrastructure

Issue #384
Remove old tests specific to the C backend
Remove old tests specific to the C backend

Issue #384
Merge

Merge branch 'remove-c-backend-384' of https://github.com/iankronquist/mypy
into iankronquist-remove-c-backend-384

Conflicts:
	mypy/build.py
	mypy/cgen.py
	mypy/icode.py
Fix type checking without running
Remove unused code
Remove unused test code
Remove unused test cases