Enumerable::ENUMERABLE_SIZE

[stepancheg]

Associated constant for the number of values in this enumerable. If the number exceeds usize::MAX, accessing the constant fails at compile time.

This is useful to create a compact storage to map all the values:

let mut data: [None; MyEnum::ENUMERABLE_SIZE];

I chose name ENUMERABLE_SIZE not SIZE to avoid confusion with possible other meaning of SIZE (for example, foru32`SIZE could also mean size in bytes).

Follow-up to this PR would be implementing:

• Enumerable::enumerable_index to map enumerable value to usize index
• Enumerable::enumerable_from_index for the opposite operation

[aarkegz]

Would you please create an issue about these features? I think that these features given/proposed in this PR are very useful and sometimes critical, but I have several questions.

• For Enumerable::ENUMERABLE_SIZE, can you come up with a better name? I think ENUMERABLE_SIZE is OK and acceptable but I'm also wondering is there a clearer name. Also I'm not sure what's the best behavior when it exceeds usize::MAX, maybe failing at compile is the best option.
• For Enumerable::enumerable_index/Enumerable::enumerable_from_index, maybe we should call them Enumerable::enumerable_until_index and Enumerable::enumerable_from_index? Also I think we should give them default implementations.

Thank you for your contribution! Very happy to see you in this repo again.

[aarkegz]

I've created issue #26 for this PR.

For Enumerable::enumerable_index and Enumerable::enumerable_from_index, I misunderstood their purpose in my last comment. I'll discuss them in #27.