psbt 0.9.0

Partially signed bitcoin transaction v0-2 library (bip174, bip370, bip371)
# 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.