Remove nonstandard 27 minimum value from ecdsa signature v
Ethereum uses 27/28 (superseded) or 35/36 for the v value. Other use
cases may want the recovery value directly. This makes it produce 0/1.
=+ [r y]=(jc-mul g k)
=/ s (pro.n `@`(inv.n k) `@`(sum.n z (mul r prv)))
=/ big-s (gte (mul 2 s) ^n)
- :* v=(add 27 (mix (end 0 1 y) ?:(big-s 1 0)))
+ :* v=(mix (end 0 1 y) ?:(big-s 1 0))
r=r
s=?.(big-s s (sub ^n s))
==
