Add BIP370 fields

[Nesopie]

This PR adds support to BIP370.

A couple of questions:

• For the input fields pertaining to hashes (eg, RIPEMD160) should I check if the preimage is the same as the hash?
• For PrevTxId - should I take the input as string or Buffer? And should I assume that the txid is already in reverse byte/little-endian order?
• should I check for correctness in general? For example, check if height locktime is less than 500 million, or hash(preimage) === hash etc or is the check function simply for type safety?
• There's a util function for writing or reading from/to Uint64. But now that Buffer has a util for performing that operation, wondering if I can use that instead?
• A lot of the existing code uses Buffer.allocUnsafe(), any reason why?

TODOS:

• Add test vectors from BIP370
• Support for a couple more global key value pairs
• Support for proprietary key value pairs
• Migrate to Uint8Array

[andrewtoth]

Is this still in progress? The PSBT_OUT_SCRIPT field is needed for silent payments.