replace usize with NonZeroUsize for LRU's capacity

[Yosi-Hezi]

There is no use for LRU with zero capacity. Using NonZeroUsize for the cap field ensures that the state of zero capacity is unreachable and simplifies the logic.

Pros:

• Simplified logic, where zero capacity is avoided. A bit easier to extend the LRU functionality.
• Improved and simplified get_or_insert API (no need to return Option)

Cons:

• Additional use statements to doc.
• Breaks previous API that uses the cap field

[jeromefroe]

Thanks for the contribution, @Yosi-Hezi!

[behzadnouri]

Can you please consider reverting this commit?
Please see #165