Avoid printing uniques in specialization rules

Akio found an avoidable cause of non-determinisim: The names of RULES
generated by Specialise had uniques in them:
    "SPEC $cshowsPrec_a2QX @ [GHC.Types.Char]" [ALWAYS] forall ...
By using showSDocForUser instead of showSDocDump when building the rule
name, this is avoided:
    "SPEC $cshowsPrec @ [Char]" [ALWAYS] forall ...
See #4012, comments 61ff.