payjoin-ffi 0.21.0

Summary

This release accommodates rust-payjoin 0.21.0, using a payjoin v2-first
API and exposing bitcoin-ffi types directly.

Changelog

• Major overhaul to attempt a stable BIP 77 protocol implementation.
• v1 support is now only available through the V2 backwards-compatible APIs.
• see payjoin-0.21.0 changelog for more details.
• Separate payjoin_ffi and payjoin_ffi::uni UniFFI types into two layers.
• Re-export bitcoin-ffi types and their UniFFI bindings

What's Changed

• Update to payjoin-0.21 by @DanGould in #36
• Use bitcoin-ffi types instead of our own bitcoin module. by @DanGould in #37
• Release payjoin-ffi-0.21.0 by @DanGould in #38

Full Changelog: v0.20.0...v0.21.0