Migrate more checker test cases to static typing

Most are still disabled.
Refactor test cases
Update test case
Update comments
Fix checking of implementation of abstract method
Update and enable test case
Update and enable test case
Update and enable test case
Update and enable test case
Update and enable test case
Update and enable test cases
Update and enable test case
Update and enable test cases
Disable unsupported test cases
Fix type test cases
Fix lexer test cases
Refactor TypeInfo

Assume that bases are never None.
Refactor to only store Instance types as bases
Partial work towards multiple inheritance

Conflicts:

	mypy/checker.py
	mypy/nodes.py
	mypy/semanal.py
Add TypeInfo.__repr__