Introduce a way to enumerate the enum values without memory allocation

[akarboush]

Fixes #494
Fixes #521

[fiseni]

We don't need to expose a new API here. Instead, we need to fix the SmartEnum.List. Right now, it reads from the _fromName dictionary and creates a brand new list for the output.
That's unnecessary, we already hold an _enumOptions field which is the list of all enums. So, we need only the following fix

public static IReadOnlyCollection<TEnum> List => _enumOptions.Value;

[akarboush]

@fiseni, you are actually correct! I completely overlooked the _enumOptions option.
I'll go ahead with it if that's alright with you, @ardalis.

[fiseni]

@akarboush @ardalis
There is more to it. I created issue #521 describing the details and the proposed changes.

[fiseni]

In the related issue, I suggested returning IReadOnlyList so the users may use also for iteration. Since it inherits from IReadOnlyCollection it won't be a breaking change.

Also, I suggested keeping the state as ReadOnlyCollection, otherwise the users will be able to cast back to an array.

Anyhow, @ardalis let's merge this PR as is. We'll tackle those details additionally.

[ardalis]

Done