core: initial implementation of https://eips.ethereum.org/EIPS/eip-1884

[holiman]

This PR implements https://eips.ethereum.org/EIPS/eip-1884 .
It adds 'Istanbul' as a new hard fork, but does not impement the full puppeth support -- we will need to know how parity and aleth defines it before we can fully add it there.
Potential TODO: refactor how we handle fork configurations, to make it

1. More flexible, so we can more easily configure what eips goes into a particular named HF,
2. More tightly coupled, so we don't risk missing a config/getter/setter in one place, or copy-paste with the wrong number

That should be in a separate PR, however.

[karalabe]

This was superseded by #19743.