Add Enum#inspect

[straight-shoota]

Implements Enum#inspect with a syntax based on Enum.[] (#12900) as proposed in #12884.

Resolves #12884

[straight-shoota]

A question is what should happen to File::Permissions#inspect. It inherits the implementation from Enum. But #to_s has a custom implementation, so that got called from Enum#inspect.

In a41e48e I'm adjusting the specs for File::Permissions#inspect to match Enum#inspect:

perm.to_s    # => "rwxrw-r-- (0o764)"
perm.inspect # => "File::Permissions[OtherRead, GroupWrite, GroupRead, OwnerExecute, OwnerWrite, OwnerRead]"

But we could also go the other way and implement File::Permissions#inspect as delegating to ``File::Permissions#to_s`.

The #to_s is more concise and still unambiguous. It's not as clearly delimited though (it's only clear when you know what format to expect) and doesn't produce a valid expression.
So I'd prefer it this way, but we could alternatively have #inspect and #to_s be the same.

[beta-ziliani]

I think it's OK to have the verbose output for File::Permissions#inspect. As a developer I might be curious as to what are the real names of a given permission's flags.