Tidy up the treatment of SPECIALISE pragmas

Tidy up the treatment of SPECIALISE pragmas

Remove the now-redundant "const-dicts" field in SpecPrag

In dsBinds, abstract over constant dictionaries in the RULE.
This avoids the creation of a redundant, duplicate, rule later
in the Specialise pass, which was happening before.

There should be no effect on performance either way, just less
duplicated code, and the compiler gets a little simpler.