Move compress() and decompress() to dedicated library crate

[gkaklas]

Hello! Thank you for this utility

These commits add some initial work so the code can be used as a library.

Mainly, the functions compress() and decompress() have been moved from main.rs to a separate crate, but there are also some changes to create a Cargo workspace to organize the crates.

I'm not sure about some details e.g. the name mozlz4-bin, but I see that, for other libraries, the usual scheme is that there is a library crate for general users, and a library-sys used by library for the unsafe functionality, so it seemed appropriate to keep the naming convention and maybe the binary name can be changed in the future.

[jusw85]

Thanks for this! I'm a little tied up at the moment so it'll be awhile before I take a look at this.