[Fulminate] Clarify Fulminate API

[ZippeyKeys12]

There are 4 parts of Fulminate's runtime library AFAIU:

1. Internals only used for runtime checking of CN stuff
2. Functions and types for evaluating CN pure terms
   1. Used by Bennet and Darcy via code generated by cn_to_ail.ml
3. Functions used by users
   1. Wildcard ownership (cn_unsafe_malloc)
4. Callbacks for use by testing
   1. cn_failure_callback

Making the interfaces clearer would be good...

[ZippeyKeys12]

Should help with #380