feature: Hide `PosixDiskEnv` type behind a feature (default enabled)

[NumberFour8]

Building rusty-leveldb in constrained or non-POSIX environments (such as WASM) is currently not possible. This PR adds small changes to make it possible.

Changes

• Introduces fs feature which is enabled by default. The feature allows to toggle the PosixDiskEnv type and therefore makes it possible to build rusty-leveldb on systems where std::io is constrained. One major use case are WASM environments. Users can therefore set use-default-features = false on rusty-leveldb dependency entries in their Cargo.toml . The only available Env will be MemEnv if fs feature is disabled.

• Updates rand crate to version 0.8.5 which is also WASM compatible. When the users of rusty-leveldb add getrandom dependency to their Cargo.toml, it will make the random number generation in rusty-leveldb work on their platform.

• Adds crate-type to allow builds in different environments

• Re-exports FileLock, RandomAccessFile and Logger types so that users can add custom implementation of the Env trait.

[dermesser]

Thank you very much! I've left a few comments, but overall it looks good.

[NumberFour8]

Thank you very much! I've left a few comments, but overall it looks good.

Thanks for the feedback :) I adapted the code according to your comments.

[NumberFour8]

@dermesser Is this good to merge, then?

[dermesser]

Yes apologies for the delay. I will shortly release a new major version