Directly linked protocols

[frenchy64]

Is the emitArgs call still appropriate for invokeStatic?

This is broken, I think because of https://clojure.atlassian.net/browse/CLJ-1865 the recursive calls needed for a protocol to lookup its own cache are not directly linked. Can we stash the cache somewhere else?

(defprotocol Foo (pfoo [this]))
(defn libfn [x] (pfoo x))
(extend-protocol Foo String (pfoo [_] :dude))
(libfn "hello")
(alter-var-root #'user/pfoo (fn [old] (fn [this] (prn :this) (old this))))
(libfn "hello")
Execution error (NullPointerException) at user/pfoo (REPL:1).
Cannot invoke "clojure.lang.MethodImplCache.fnFor(java.lang.Class)" because "cache__8012__auto__2146" is null

Idea: start from scratch, this time try and broaden the Fn's that canBeDirect. For protocols, we need to:

1. allow (.__methodImplCache this) without counting towards "this" usages, and
2. allow functions to close over other directly linkable functions.