a577c22b12f4edf0b2f2f266fb13656caf1d21c4 fix(persist): add default feature to enable bdk_chain/std (Steve Myers)
Pull request description:
### Description
This PR adds a `default` feature to `bdk_persist` so it can be build on its own. Once #1422 is done we can remove the `default`again.
### Notes to the reviewers
I need to be able to build `bdk_persist` on its own so I can publish it to crates.io.
### Checklists
#### All Submissions:
* [x] I've signed all my commits
* [x] I followed the [contribution guidelines](https://github.com/bitcoindevkit/bdk/blob/master/CONTRIBUTING.md)
* [x] I ran `cargo fmt` and `cargo clippy` before committing
ACKs for top commit:
ValuedMammal:
ACK a577c22b12f4edf0b2f2f266fb13656caf1d21c4
oleonardolima:
ACK a577c22b12f4edf0b2f2f266fb13656caf1d21c4
storopoli:
ACK a577c22b12f4edf0b2f2f266fb13656caf1d21c4
Tree-SHA512: 8b07a9e4974dec8812ca19ce7226dcaece064270a0be8b83d3c326fdf1e89b051eb0bd8aa0eda9362b2c8233ecd6003b70c92ee046603973d8d40611418c3841
66abc73c3d