introduce secp operators

[arvidn]

This is a softfork activated by ENABLE_SECP_OPERATORS. It's based on @cameroncooper 's patch which implements verification of secp256k1 and secp256r1 signatures.

The two new operators are:

name	opcode	cost	description
secp256k1_verify	0x0cf84f00	850000	Verifies a signature given a public key, message digest and signature
secp256r1_verify	0x1c3a8f00	1850000	Verifies a signature given a public key, message digest and signature

The message digest parameter must be the sha256 of the message to sign.

This is best reviewed one commit at a time.

[coveralls-official]

Pull Request Test Coverage Report for Build 5203783675

• 60 of 65 (92.31%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.3%) to 92.837%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/chia_dialect.rs	12	13	92.31%
src/secp_ops.rs	45	46	97.83%
src/f_table.rs	0	3	0.0%

Totals
Change from base Build 5203270196:	0.3%
Covered Lines:	5042
Relevant Lines:	5431

---

💛 - Coveralls

[arvidn]

@rostislav pointed out that there's a reason for k1 to be about twice as fast as r1. so maybe it would make sense to reflect that in the cost

[richardkiss]

Seems good

[richardkiss]

I haven't looked too closely since last time. But I thought I'd approve it anyway since my understanding is only the costs have changed.