BLS12-381 curve operations (ethereum#2537)

* draft

* Update eip-x.md

* add ABI

* also mention encoding of boolean vars in pairing

* add point decompression

* fix decompression, add gas price

* expand on square root extraction and checks

* Cleanup title

* Fix typo and remove optional header fields

* set EIP number

* hm, html verifier is not satisfied...

* update on ABI, costs and subgroup checks

* add more information about field-to-curve

* add links to implementations

* spellcheck

* mapping costs are underetmined yet

* add mapping operations cost

* whoops, spellcheck!

* updates: test vectors, prefixes

* more explicit pairing output

* gas consumption in case of error

* fix multiplication -> multiexp

* split mapping into two separate ones

* spellcheck

* update link to Go implementation

* add addresses

Co-authored-by: Kobi Gurkan <kobigurk@gmail.com>