Revert 6f68e52 and fix the gcc2 build via template.

* Instead of forcing the hash-table to use a copy of the key,
introduce and use TypeOperation template to avoid taking a
reference of a refere

Revert 6f68e52 and fix the gcc2 build via template.

* Instead of forcing the hash-table to use a copy of the key,
introduce and use TypeOperation template to avoid taking a
reference of a reference type (which gcc2 doesn't allow).

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