PSBT implementation

Implements both v0 (BIP-174) and v2 (BIP-370) versions of PSBT specification.

Based on bitcoin PSBT implementation, but wraps it into new type system supporting v2 features and providing convenient functions to iterate over sets of transaction inputs/outputs and corresponding PSBT key maps.

lib.rs:

PSBT bitcoin library, providing all PSBT functionality from [bitcoin] library, plus

• constructor, supporting miniscript-based descriptors, input descriptors, all sighash types, spendings from P2C, S2C-tweaked inputs ([construct]);
• advanced signer, supporting pre-segwit, bare and nested segwit v0, taproot key and path spendings, different forms of tweaks & commitments, all sighash types ([sign]);
• commitment-related features: managing tapret-, P2C and S2C-related proprietary keys;
• utility methods for fee computing, lexicographic reordering etc;
• command-line utility for editing PSBT data (WIP).