[Truffle] Rename CaseEqualNode to SameOrEqualNode to reflect semantics.
[Truffle] Document SameOrEqualNode.
[Truffle] Get rid of superfluous #=== and #eql? definitions.

* So we match MRI and avoid code duplication.
* MRI still defines #=== uselessly on a couple classes like Fixnum, Symbol and String.
* No more reference to the confusing "three equal".