Remove releases, introduce min version testing, match rust-bitcoin lint and fmt

[nyonson]

Without the releases dependency graph, pretty easy to dogfood min and recent version testing. Also easier to adopt rust-bitcion's lint and format rules. I kept it a workspace even though it only has one package to dogfood the slightly more complex case. Plus, might add some packages in the future.

Closes #54

[tcharding]

ACK 04f8b0a

[apoelstra]

In e195b5a:

This fails to restore the existing lockfile if LockFile::Minimal.derive or LockFile::Recent.derive fail. We should use a drop guard here to ensure that restore_existing is run no matter how the function exits (or we should manually call restore_existing in each error path, which in this case might be fewer LOC).

[apoelstra]

We could change backup_existing to return a handle which calls restore_existing on Drop.

[apoelstra]

Otherwise 04f8b0a looks good. We should add avoid-breaking-exported-api = false to clippy.toml here because there is no public API we're exposing.