BIP0174 PSBT Support

[MatthewLM]

Create a new PartiallySignedTransaction class that allows creation, updating and signing of transactions in the PSBT BIP0174 format.

• Create classes for the underlying PSBT data structure: PSBTKeyValue, PSBTMap as Writable classes.
• Create abstract PSBT class for overall data structure: PartiallySignedTransaction. Include interface for updating input/output data, signing, merging, finalizing and obtaining the complete transaction.
• Add write method to PartiallySignedTransaction.
• Create PartiallySignedTransactionV1 to represent the BIP0174 PSBT which is technically v0 but often regarded as v1. Include a create named constructor.
• Implement update methods.
• Implement signing.
• Implement merging.
• Implement finalization.
• Implement transaction construction.
• Add a fromReader factory constructor to PartiallySignedTransaction to check for and return the relevant sub-class: only PartiallySignedTransactionV1 to begin with.
• Add all test vectors for BIP0174.