Register a primitive function for use in rewriting calls of a trigger
function in FGL.
This is invoked as follows:
(add-fgl-primitive trigger-fn primitive-fn)
This invocation adds an entry in the fgl-rewrite-rules table that
allows primitive-fn to be tried when symbolically interpreting
trigger-fn invocations. For this to work, primitive-fn must be
defined as an FGL primitive and installed in the current attachment for
fgl-primitive-fncall-stub using the install-fgl-metafns event.