P2SH and Version 0 Segwit Script enforcement from genesis

Hi,

I propose backdating the P2SH and Segwit version 0 script rules back to the genesis block, as a way to simplify these consensus rules. Here's the abstract from a draft BIP I wrote up to explain this change:

The Pay to Script Hash (P2SH, BIP 16) script rules and the Version 0 Witness Program script rules (BIP 143/141) can be enforced from the genesis block with only one historical exception. Doing so simplifies consensus rules and allows protocol implementers to avoid writing and testing code paths that are no longer relevant.

The full BIP draft can be found here: https://github.com/sdaftuar/bips/blob/p2sh-v0segwit-from-genesis/bip-sdaftuar-p2sh-v0segwit-from-genesis.mediawiki

And the currently open pull request to Bitcoin Core which implements this change can be found here: https://github.com/bitcoin/bitcoin/pull/11739