Support transform for field assignments

[athos]

With the current implementation of +++, the result of macro expansion for field assignments looks somewhat problematic:

cljs.user=> (def obj #js{:x 0})
#'cljs.user/obj
cljs.user=> (+++ (set! (.-x obj) 42))
clojure.lang.ExceptionInfo: set! target must be a field or a symbol naming a var
 at line 1 <cljs repl> {:file "<cljs repl>", :line 1, :column 1, :root-source-in
fo {:source-type :fragment, :source-form (+++ (set! (.-x obj) 42))}, :tag :cljs/
analysis-error}
        at clojure.core$ex_info.invokeStatic(core.clj:4617)
        at clojure.core$ex_info.invoke(core.clj:4617)
        at cljs.analyzer$error.invokeStatic(analyzer.cljc:632)
...
cljs.user=> (macroexpand '(+++ (set! (.-x obj) 42)))
(do (set! (fence.core/dot obj -x) 42))
cljs.user=>

(The form (set! (fence.core/dot obj -x) 42) will in turn be expanded to (set! (aget obj "x") 42), which is a bad form.)

This PR enables +++ macro to deal with field assignment forms appropriately.

cljs.user=> (+++ (set! (.-x obj) 42))
42
cljs.user=> (macroexpand '(+++ (set! (.-x obj) 42)))
(do (clojure.core/aset obj "x" 42))
cljs.user=>

Also, a new macro fence.core/set! is provided as a shorthand for the combination of +++ and set!:

cljs.user=> (fence/set! (.-x obj) 42)
42
cljs.user=> (macroexpand-1 '(fence/set! (.-x obj) 42))
(clojure.core/aset obj "x" 42)
cljs.user=>