Keep String keys distinct in a compare_by_identity Hash - #2802

Fixes #2615.

What

A String key was unwrapped to its primitive value (key.valueOf()) before being stored in the hash's underlying Map, so a compare_by_identity hash keyed such entries by value. A duped key then found the entry stored under the original:

str1 = "foo"
str2 = str1.dup
h = {}.compare_by_identity
h[str1] = 1
p [h[str1], h[str2]]  # Opal: [1, 1] — MRI: [1, nil]

Why this fix

When the hash compares by identity, the String object is now used as the Map key as-is. Map compares keys with SameValueZero, which is exactly the identity semantics wanted here — so no separate identity table is needed. Consequently such keys also get no entry in $$keys: indexing them there by value would defeat the property being preserved. hash_put, hash_get, hash_delete and hash_rehash are updated consistently.

Note this deliberately does not route string keys through Opal.id: that raises on a primitive string (Cannot create property '$$id' on string), since $prop cannot define a property on a primitive under use_strict.

Scope / known limitation

This fixes String objects — those from dup, clone, etc., which have real identity.

It does not fix equal string literals, which Opal compiles to the same JS primitive: 'foo'.equal?('foo') is already true and object_id is identical, independent of Hash. Distinguishing two literals requires changing how string literals are compiled, so "Hash#compare_by_identity gives different identity for string literals" stays filtered, now with that as the reason.

Testing

• Unfiltered 2 previously failing specs — Set#compare_by_identity regards #dup'd objects as having different identities and the #clone'd equivalent — which this fixes for free, as Set is Hash-backed.
• Full rake mspec_node green: 14626 + 693 examples, 0 failures.
• Verified no regressions by running hash, string, array and set suites against a stashed tree and comparing example and failure counts: Hash 532/4 → 532/4 (all ruby2_keywords_hash, pre-existing), String 1362/7 unchanged, Array 1269/32 unchanged, Set 264/14 → 268/14 (+4 examples from the unfiltered specs).
• Each behavior change was checked against MRI directly, including the corner case of calling compare_by_identity on an already-populated hash (moves toward MRI: [1, 2, 2] → [1, 2, nil], MRI [1, nil, nil]).

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