- there were previously no hashing mechanisms for lists so they
would fall back to the definition for primitives and hash the
memory location of the allocated List object.
- provide a UList::Hash<> sub-class for inheritance, and also a global
specialization for UList<T>, List<T> such that the hash value for
List<List<T>> cascades properly.
- provide similar function in triFace to ensure that it remains
similar in behaviour to face.
- added SymmHash to Pair, for use when order is unimportant.
STYLE: use string::hash() more consistently
- no particular reason to use Hash<word>() which forwards to
string::hash() anyhow
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