Adds Within trait and fixes more Contains edge cases

[michaelkirk]

• [x] I agree to follow the project's code of conduct.
• [x] I added an entry to CHANGES.md if knowledge of this change could be valuable to users.

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~~Based on #880, so review that first.~~ Merged! This is ready for review!

The new Within trait is implemented in terms of Contains.

Running the "within" tests from the JTS suite exposed some further inconsistencies with how we're handling line string edge cases.

[michaelkirk]

Now that https://github.com/georust/geo/pull/880 has been merged, this is ready for review!

[michaelkirk]

bors r=frewsxcv

[bors[bot]]

Build succeeded:

• ci result