Rename Waf to WAF

[anuraaga]

In Go, it is customary for types to be named in all caps when they are acronyms, unlike many other languages. e.g., URL. It would be more idiomatic for our type to be named WAF instead of the current Waf.

This is a huge change so it could be considered ship has already sailed, but if doing it then before v3 is probably the time to do it.