Refactor contours code

[theotherphil]

@lazareviczoran this refactors some of your recent additions to this library.

In particular, it:

• Extracts code for performing calculations using points and lines into a new module
• Extracts code for computing convex hulls, bounding rectangles, arc length, and polygon approximation into a new module
• Removes unneccesary type bounds from the Point type and several functions
• Introduces a BorderType enum in place of is_outer
• Improves doc comments
• Refactors the implementation of find_contours_with_threshold to improve readability

Let me know if there's anything here that you disagree with or that you believe is incorrect.

[theotherphil]

Thanks for the review. Good suggestions - I've updated this PR to incorporate them. I'll merge this after CI goes green, and will then release a new version of the crate.