Fork `rusty-leveldb` to make it compatible with `bedrock-rs`

[RadiatedMonkey]

Currently the rusty-leveldb crate has several issues that make it difficult to use in our project. Firstly, the library is not thread safe which means every operation must lock the global database and secondly, all buffers returned by the library uses the Bytes type from the bytes crate, which is incompatible with our methods.

Other smaller issues I've noticed were the filter policy option using an Rc<Box<dyn FilterPolicy>> type while Rc<dyn FilterPolicy> should suffice. I assume the intention was to create a filter policy trait object but they did not realise that Rc also supports trait objects.

The repo has already been forked at leveldb-rs.