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