Remove implementation detail specs for Enumerator

[Earlopain]

Enumerator::Yielder, Enumerator::Generator and Enumerator::Producer are not supposed to be exposed to the user.
Instead they will interact with them trough other means, like for example Enumerator.new { |yielder| }.

Some specs I ported, others I removed, and some were already present.

I also have this PR to nodoc them ruby/ruby#17036 (they are already basically just present by name only)

[eregon]

I agree they shouldn't be documented (and merged you ruby/ruby PR) but they are still public API I would say, and based on that I think it's fair enough to test them in ruby/spec.
The are exposed to the user, e.g.:

$ ruby -e 'Enumerator.new { |y| p y }.take(1)'
#<Enumerator::Yielder:0x00007fac648b5a48>

So I wouldn't call the class names implementation details since they are public constants.

Is there something specific these specs test you think they shouldn't?

From a quick look core/enumerator/generator/initialize_spec.rb and core/enumerator/yielder/initialize_spec.rb seem rather useless except the first one checks initialize can't be called on a frozen instance which is something.
The rest looks not too unreasonable to me from a quick look.

[eregon]

In general the philosophy of core specs is "spec every public method of every public class/module" (and also some private methods like initialize). It's actually what mkspec does.

[eregon]

I do like the new specs though, so maybe we'd only remove the useless initialize specs but keep the rest?