Implemented iterator for partitions

[jacksonhenry3]

Summary:

This pull request impls the iterator trait for the Partitions struct so it can be used as an iterator directly.

Description:

• Added an iterator implementation to the Partitions struct.
• The iterator yields Vec<usize>, where each vec represents a partition.

Testing:

Tested against the existing oeis test case, ensuring that the iterator still yields the expected partitions for the first few entries of A000041.

[colt-browning]

Win in usability, but loss in performance: this implementation allocates a new vector at each iteration.
(Just pointing this out, not implying that this is good or bad.)