Revisit how we implement glacier/bpo forks

[SamWilsn]

Currently, constant-only hardforks (aka glacier and BPO forks) are full forks with their own directory in src/ethereum/forks/..., and they have special privileges as far as code coverage and ethereum-spec-lint go.

@petertdavies wondered whether these forks deserve a full copy of the code, and should instead be replaced with a table somewhere.

[SamWilsn]

	Pros	Cons
Fork as Folder	Simplicity Uniformity	Repetitive Maintenance burden
Table of Forks	Less Code Easier Refactoring	Special Case Inconsistent public API