+sign:schnorr crashes on `=(0 sk)`, so the bounds checking code is not
exercised for sk=0. It also crashes on `(gte sk n.domain.c)`, which is
redundant with the size check on sk, so we remove that.
The secp core had some flaws: in particular, the logic for signing/recovery
did not match libsecbp256k1 w.r.t. the enigmatic "recid" (v) value. The jet
hints were also subtly wrong, in that the curve parameters were in a sample
(not an arm) and thus not matched by the jet matching scheme. Consequently,
the jets would be used (but incorrect) for other curve parameters.
Tests were also added to exercise the recovery id cases thoroughly.
