Fix HashDict so that key comparison uses exact match

This change means that HashDict buckets are nolonger strictly ordered.
Previously keys which compared equal (==) but not exactly equal (===)
would be considered different for large sizes but equivalent for small
sizes. This ambiguity occured because hashes are only equal when terms
are exactly equal (===), so both now use exact (===) key comparisons.
Small HashDicts are still a single ordered list and large HashDicts
contain ordered buckets. Both of these may contain keys that compare
equal (==) but are not exactly equal (===), should these keys be present
in the same list or bucket the ordering of these keys (relative to each
other) is undefined.

See github issue #1171 for examples.

Note: If assumptions were made about the form of a HashDict they may
nolonger be valid. Particularly that HashDicts can nolonger be reliably
pattern matched on for equality.