Bump rust-bitcoin to 0.29

[danielabrozzoni]

I had to revert 91b8e3c for 0.2.3 (we needed the update in bdk, and bdk is still stuck at 0.28). Re-revert the commit for the next release.

[notmandatory]

@danielabrozzoni I think BDK is almost ready for this one now that rust-miniscript 0.8 is release with rust-bitcoin 0.29. Are you already working on a PR?

[danielabrozzoni]

I just have to re-apply 91b8e3c :)