Pure Rust implementation

[semiexp]

Currently cspuz_core depends on SAT solvers written in C++, making it incompatible with WebAssembly build tools such as wasm-pack. It would be good if we can add support for a SAT solver in pure Rust.

Candidate SAT solvers:

• splr

TODO

• measure performances of candidate solvers
• choose SAT solver to use
• add minimal support
• support custom propagators