A draft proposal for a new Special Functions chapter.

Initial draft. Feedback welcome.

I wanted to collect descriptions of compiler-generated functions in one place for easy reference, but some of this material is redundant with material already present in various places in the spec.  So one way to integrate this with the spec would be to turn it into an appendix with cross-references, and then ensure that all of the important content was integrated (duplicated) in the body of the spec.