lean-sys update

Stream: lean4

[Jad Ghalayini]

Hello! After a lengthy disappearance caused by my PhD catching up with me, I've recently updated lean-sys to version 0.3.0 to track the latest changes to lean.h. The version bump from 0.2.0 is due to the breaking change of removing the fixpoint family of functions, as in Lean 4 commit f9f074d. Hopefully I'll have times to continue my project by publishing some higher-level bindings to Lean, with the end goal of seamless integration between Lean 4 and Rust code, as I'll need for my planned thesis. If anyone has any ideas/feature requests/found bugs/etc, I'd be happy to hear!