Introduce the strongly typed enum BlockValidity to represent the validity of a block

Summary: Use it as parameter to IsValid and RaiseValidity. The type safety allows us to remove some asserts as they are now invariant of the type system.

Test Plan:

make check
./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, schancel

Reviewed By: #bitcoin_abc, schancel

Subscribers: schancel, teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1445