fix: looser type for allowedSighash

[kyranjamie]

SignatureHash is the three sig hash values, a default, and the ANYONECANPAY modifier.

As far as I understand, the sighash can be any of the possible 6 resulting combinations [0x01, 0x02, 0x03, 0x80, 0x81, 0x82, 0x83], meaning this type on signIdx is too strict, as say it doesn't except, say 0x82.

In this PR I've just loosened the type, but maybe library should check for these possible 6 values, either as a type or runtime check?

Or, might it make sense to update the enum?

export enum SigHash {
  ALL = 0x01,
  NONE = 0x02,
  SINGLE = 0x03,
  ALL_ANYONECANPAY = 0x81,
  NONE_ANYONECANPAY = 0x82,
  SINGLE_ANYONECANPAY = 0x83,
}

cc/ @fbwoolf

[paulmillr]

Thanks, incorporated in 07581a7