Drop isPost* method

[twoeths]

in typescript we use isPost*() a lot

@spiral-ladder prefer using enum with (>, <, ==) instead thanks to @ intFromEnum util

refer to #28 (comment)

[spiral-ladder]

Left a comment under that comment:

This is a good point, it's also easy to miss if it's just a single operator. For safety it might be better to keep it this way then.

[twoeths]

one more point is unlike typescript, BeaconState in zig implementation is fork-aware https://github.com/ChainSafe/state-transition-z/pull/17/files#diff-f3c8638ffff8c55ffd2c70324cc540954a42662eefe344195c145adb099b1f33R208, we can do BeaconState.isPostElectra(). Having fork.isPostElectra() is compatible to it.