cache and reuse scanned enum options

[norvegec]

Since GetAllOptions() method

• performs assembly scan
• is private
• used 3 times in the class
  it makes sense to cache it's result.

[norvegec]

Could be superseded with #100

[chaughawout]

#104 might benefit from some details here.

[norvegec]

This PR

• contains non-functional, performance improvements
• doesn't contain breaking changes
• all tests are passed
  @ardalis what else is required to move forward with it? Since you are the author and maintainer, could you give some feedback?

[ardalis]

Sorry - maintaining a bunch of projects and this one has fallen behind in my rotation of keeping things updated.

[ardalis]

Thanks! Will try to get a new release out with this fix soon.