Merge #13666: Always create signatures with Low R values

e306be742932d4ea5aca0ea4768e54b2fc3dc6a0 Use 72 byte dummy signatures when watching only inputs may be used (Andrew Chow)
48b1473c898129a99212e2db36c61cf93625ea17 Use 71 byte signature for DUMMY_SIGNATURE_CREATOR (Andrew Chow)
18dfea0dd082af18dfb02981b7ee1cd44d514388 Always create 70 byte signatures with low R values (Andrew Chow)

Pull request description:

  When creating signatures for transactions, always make one which has a 32 byte or smaller R and 32 byte or smaller S value. This results in signatures that are always less than 71 bytes (32 byte R + 32 byte S + 6 bytes DER + 1 byte sighash) with low R values. In most cases, the signature will be 71 bytes.

  Because R is not mutable in the same way that S is, a low R value can only be found by trying different nonces. RFC 6979 for deterministic nonce generation has the option to specify additional entropy, so we simply use that and add a uin32_t counter which we increment in order to try different nonces. Nonces are sill deterministically generated as the nonce used will the be the first one where the counter results in a nonce that results in a low R value. Because different nonces need to be tried, time to produce a signature does increase. On average, it takes twice as long to make a signature as two signatures need to be created, on average, to find one with a low R.

  Having a fixed size signature makes size calculations easier and also saves half a byte of transaction size, on average.

  DUMMY_SIGNATURE_CREATOR has been modified to produce 71 byte dummy signatures instead of 72 byte signatures.

Tree-SHA512: 3cd791505126ce92da7c631856a97ba0b59e87d9c132feff6e0eef1dc47768e81fbb38bfbe970371bedf9714b7f61a13a5fe9f30f962c81734092a4d19a4ef33

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