mutable deftype fields: (set! field val) in a method

deftype fields tagged ^:unsynchronized-mutable / ^:volatile-mutable can now be
reassigned in place from a method, as on the JVM. A jrec stores fields as cons
cells, so a new jolt-set-field! mutates the pair with set-cdr!. The deftype macro
rewrites (set! mutable-field v) in a method body to (set! (.-field inst) v), and
the analyzer compiles a (set! (.-field obj) v) target to jolt-set-field! — so
both the rewritten symbol form and an explicit interop (set! (.-root this) v) go
through one path. Field reads remain a snapshot at method entry, which is correct
for the universal read-then-set pattern (a repeated set! of the same field in one
call would read the entry value).

Closes the set!-of-local SCI failures: SCI load 202 -> 205/218.

jolt-core/jolt/ir.clj

@@ -97,6 +97,7 @@
                                  :ret (f (get node :ret)))
       (= op :throw)  (assoc node :expr (f (get node :expr)))
       (= op :set-var) (assoc node :val (f (get node :val)))
+      (= op :set-field) (assoc node :obj (f (get node :obj)) :val (f (get node :val)))
       (= op :defmacro) (assoc node :fn (f (get node :fn)))
       (= op :invoke) (assoc node :fn (f (get node :fn))
                                  :args (mapv f (get node :args)))