Small refactors of Cargo.toml file

[thunderbiscuit]

I've been looking at our build process for a few days now and getting into all sort of rabbit holes. This PR does 2 things to bring the Cargo.toml file in line with the uniffi documentation on correct uniffi project setup:

1. Remove the default feature and simply bring it on the uniffi dependency. This is just a way to make more explicit where the feature is turned on.
2. Remove the lib type of crate type. This is because it is not required as no projects depend on bdk-ffi directly. It therefore adds an unnecessary burden to our build times by producing an artifact we don't need.

The second commit just runs cargo update and commits the update to the dependency tree in the lock file.

Note: merge #858 first.

[reez]

ACK 4bad63f

Good alignment w uniffi docs, and this is cool Remove the lib type of crate type