Use ident hash for top-level recursion check

We track recursion in order to not infinite loop in ==, inspect, and
similar methods by keeping a thread-local 1 or 2 level hash. This allows
us to track when we have seen the same object (ex. using inspect) or
same pair of objects (ex. using ==) in this stack before and to treat
that differently.

Previously both levels of this Hash used the object's memory_id as a key
(using object_id would be slow and wasteful). Unfortunately, prettyprint
(pp.rb) uses this thread local variable to "pretend" to be inspect and
inherit its same recursion behaviour.

This commit changes the top-level hash to be an identity hash and to use
objects as keys instead of their object_ids.

I'd like to have also converted the 2nd level hash to an ident hash, but
it would have prevented an optimization which avoids allocating a 2nd
level hash for only a single element, which we want to keep because it's
by far the most common case.

So the new format of this hash is:

{ object => true } (not paired)
{ lhs_object => rhs_object_memory_id } (paired, single object)
{ lhs_object => { rhs_object_memory_id => true, ... } } (paired, many objects)

We must also update pp.rb to match this (using identity hashes).

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