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