lmdb::Error and semantic versioning

[dfoxfranke]

The lmdb Error type is exposed via rkv::error::StoreError::LmdbError. rkv doesn't re-export the lmdb-rkv crate, so anyone who wants to use this type has to make lmdb-rkv a direct dependency of their own crate. This makes any change to lmdb-rkv version constraint in rkv's Cargo.toml a breaking change which requires a semantic version bump in rkv.The release of rkv-0.10.4 bumped the lmdb-rkv dependency to 0.14, which broke my code since my own Cargo.toml was still depending on 0.12.

n.b., fixing #186 would eliminate my need to use this type directly.