Do occurrence analysis on result of BuiltInRule

Authored by simonpj.

Description

Do occurrence analysis on result of BuiltInRule

Previously we did occurrence analysis on the result of a
non-built-in RULE, but not of a built-in one. It makes a
difference if the rule returns something with binders
(which admittedly it usually does not). I'm about to
introduce just such a rule for 'seq'.