Implement DBMs/CDDs and or Federations in Rust

[seblund]

Allows us to lose the dependency to UDBM which makes the project run entirely in rust!
This has many benefits:

• Ensuring safe code as FFI is always unsafe
• Allowing multithreading for performance increases (UDBM is single-threaded so we use a global lock to block multithreaded access)
• Easier debugging as FFI is prone to segmentation faults which we cannot catch from rust
• Simplifying the build process by losing the dependency to UDBM/FFI
• Potentially allows us to perform use case specific optimizations of the internals

We can determine the methods that must be ported to rust from wrapper.cpp, and we have all source code available so we should be able to port without much trouble. I suspect that no code outside of src/DBMLib will need to be changed to handle a fully rust-based implementation.

Ulrik (edit): An (partial) implementation of DBM in Rust is available at: https://github.com/ScumbagDog/rdbm