[Bug #20253]

All the way down to Ruby 1.9, `Proc`, `Method`, `UnboundMethod`
and `Binding` always had their own specific clone and dup routine.

This caused various discrepancies with how other objects behave
on `dup` and `clone. [Bug #20250], [Bug #20253].

This commit get rid of `CLONESETUP` and use the the same codepath
as all other types, so ensure consistency.

NB: It's still not accepting the `freeze` keyword argument on `clone`.

Co-Authored-By: Étienne Barrié <etienne.barrie@gmail.com>

Previously, due to a change to fix bug 15608, Method#inspect output
changed for class methods:

Ruby 2.7
"#<Method: String.prepend(*)>"

Before change:
"#<Method: #<Class:Object>(Module)#prepend(*)>"

This is wrong because the Method object was created from String and
not Object. This is because the fix for bug 15608 assumed it was
being called on the singleton class of a instance, and would skip
the first singleton class until it got to the class itself.  For
class methods, this results in always using the superclass. Fix
behavior to not skip until the superclass if the singleton class
is the singleton class of a module or class.

After change:
"#<Method: #<Class:Object>(Module)#prepend(*)>"

Fixes [Bug #17428]

Previously, if an object has a singleton class, and you call
Object#method on the object, the resulting string would include
the object's singleton class, even though the method was not
defined in the singleton class.

Change this so the we only show the singleton class if the method
is defined in the singleton class.

Fixes [Bug #15608]