Add Fork Choice compliance test generator

[ericsson49]

An implementation of the Fork Choice compliance test generator, which is a part of the initial phase of Fork Choice compliance testing methodology.

More information, including links to generated test suites, is available in the preliminary announcement.

This work was supported by a grant from the Ethereum Foundation.

Test suites

Test suite	size	Purpose	Link
Tiny	135 tests	Demonstration, smoke testing	link
Small	1472 tests	Initial adoption, smoke testing	link
Standard	13240 tests	Main testing	link

TODO list

• [ ] Code refactoring
• [ ] Decide where to put the test generator, as it's not supposed to be a part of regular runs
• [ ] Decide how to integrate the test generator into existing release procedures