Moving the `is_post_xxx` functions to a separate module allows `genesis`
to also use them (cyclic import from `context` prevented this before).
This allows removing `FORKS_BEFORE_ALTAIR` and `FORKS_BEFORE_BELLATRIX`
constants and adding a more general `is_post_fork` function that needs
less maintenance. This then allows definition of `with_all_phases_from`
to streamline the implementation of the `with_xxx_and_later` decorators.

Co-authored-by: Hsiao-Wei Wang <hsiaowei.eth@gmail.com>