Candidate v0.8.9 changes

[jgriffiths]

• Update libsecp to the latest version
• Support both full and partial master blinding keys
• Support for deterministic blinding factor generation matching Jade and Ledger
• Add a rangeproof size estimation helper
• Documentation cleanups
• Bump version to v0.8.9 for release

[LeoComandini]

utACK 9f2f42df357f5b76d6273ab3165fb9ca671841d8, code review

[JamieDriver]

utack 9f2f42df (did some minimal manual testing of hashprevouts & bfs - lgtm)