Add Rust implementation of core interval operations for performance

[avaitla]

This adds a high-performance Rust backend for portion that can be used to speed up interval operations by 10-1000x depending on the operation.

Key changes:

• rust_core/: PyO3-based Rust implementation of Interval operations

  • BoundType enum (Closed/Open)
  • Value enum (NegInf/Finite/PosInf)
  • Atomic struct for single intervals
  • Interval struct for union of atomic intervals
  • All core operations: union, intersection, complement, difference
  • Python bindings with RustInterval and helper functions

• benchmark.py: Performance comparison script showing speedups:

  • Interval creation: 22-40x faster
  • Intersection: 850-1100x faster
  • Union: 9-42x faster
  • Complement: 430-800x faster
  • Containment checks: 12-15x faster
  • Difference: 740-1090x faster
  • Real-world mixed operations: 50-52x faster

Build and install with:
cd rust_core && maturin build --release
pip install target/wheels/*.whl

Usage:
import portion_rust as PR
interval = PR.rust_closed(0, 10) | PR.rust_closed(20, 30)

[AlexandreDecan]

Any human comment on this?

[avaitla]

Thanks for the work on this it's very useful for us. Apologies it was a mistake to open this PR back to the main repo. We use this library but have a lot of intervals and found the bottleneck to be here. I've been implementing in rust we only need date time intervals and didn't mean to make a pr back into main branch.