Avoid using heap on storing names, description, and hint

[rhysd]

Currently all options' short/long names, description, and hint are allocated on heap memory when building OptGroup instance. However, in almost all cases, they are given as &'static str so those allocations are not necessary.

This PR avoids those allocations which are unnecessary in almost all cases by introducing new lifetime 'a.

[tgross35]

I don't know if this is worth it because it's pretty much a one-time cost, and this comes with some usability reduction. It's also alloc-free for empty strings. We could instead take impl Into<Cow<'static, str>> in the signatures (over an API break) to save the allocations without taking on a lifetime, but again that doesn't seem worth the complexity.

There are a few alloc-related improvements that I do think make sense:

• Store an Option<char> rather than String for the short name since we should be rejecting >1 char anyway.
• Options::usage_with_format could probably be deprecated and replaced with something that takes &dyn core::fmt::Write. It shouldn't need to allocate a fresh string for each row.
• Basically all API on Matches that needs to return a string returns String but they could be &str or -> impl Iterator<Item = ...>. (that would be an API break)