replace old libsecp256k1 fork with upstream; boosts k1 recovery performance

[spoonincode]

Change Description

Replace usage of an old libsecp256k1 fork with latest upstream. This allows us to make use of (recently expired patented) improvements boosting key recovery performance. My measurements of the improvement:

CPU	Previous	New
Intel 9600k	50µs	37µs
AMD 3950X	60µs	43µs

(exact performance numbers will vary based on hardware and many other factors; these numbers we gathered with only a single core loaded)

Okay, full disclosure: the old secp256k1 fork we were using could have the patented code enabled (and the performance improvement) with a compile time option. However, I'm taking this opportunity to upgrade us to the upstream mainline code instead of an old fork. Some users have voiced concern over using an aging non-upstream fork (#8984) and while I don't think there is any reasonable chance that Cryptonomex's repos will disappear, secp256k1's main upstream repo almost certainly won't.

Needs EOSIO/fc#180, where much of the changes actually are

Change Type

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• Documentation

• Stability bug fix

• Other

• Other - special case

Testing Changes

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• New Tests

• Existing Tests

• Test Framework

• CI System

• Other

Consensus Changes

• Consensus Changes

API Changes

• API Changes

Documentation Additions

• Documentation Additions

[spoonincode]

Hey guys, we really need to approve and merge EOSIO/fc#180 first. Otherwise, now develop's submodule is no longer pointing to fc's eosio branch