Add "registers" for pod data; add send! function (see #339)

Test case:

    #!/usr/bin/env boot
    (require '[boot.pod :as pod])
    (def p (pod/make-pod))
    (def x (into [] (range 0 1000000)))
    (prn (pod/with-eval-in p (count ~x)))

This fails with "RuntimeException: Class file too large!"

    #!/usr/bin/env boot
    (require '[boot.pod :as pod])
    (def p (pod/make-pod))
    (def x (into [] (range 0 1000000)))
    (prn (pod/with-eval-in p (count ~(pod/send! x))))

This works, no problem.

boot/base/src/main/java/boot/App.java

@@ -49,8 +49,22 @@ public class App {
     public  static       String             getBootVersion()    { return bootversion; }
     public  static       String             getClojureName()    { return cljname; }
 
-    private static final WeakHashMap<ClojureRuntimeShim, Object> pods = new WeakHashMap<>();
-    public  static       WeakHashMap<ClojureRuntimeShim, Object> getPods() { return pods; }
+    private static final ConcurrentHashMap<String, String>       podRegisters = new ConcurrentHashMap<>();
+    private static final WeakHashMap<ClojureRuntimeShim, Object> pods         = new WeakHashMap<>();
+
+    public static WeakHashMap<ClojureRuntimeShim, Object>
+    getPods() {
+        return pods; }
+
+    public static String
+    getRegister(String name) throws Exception {
+        String ret = podRegisters.get(name);
+        podRegisters.remove(name);
+        return ret; }
+
+    public static void
+    setRegister(String name, String value) throws Exception {
+        podRegisters.put(name, value); }
 
     public static class
     Exit extends Exception {

boot/pod/src/boot/pod.clj

@@ -12,7 +12,7 @@
     [dynapath.dynamic-classpath :as cp])
   (:import
     [java.util.jar        JarFile]
-    [java.util            Properties]
+    [java.util            Properties UUID]
     [java.net             URL URLClassLoader URLConnection]
     [java.util.concurrent ConcurrentLinkedQueue LinkedBlockingQueue TimeUnit]
     [java.io              File]
@@ -182,6 +182,14 @@
     (.offer @shutdown-hooks f)
     (->> f Thread. (.addShutdownHook (Runtime/getRuntime)))))
 
+(defn send!
+  "This is ALPHA status, it may change, be renamed, or removed."
+  [form]
+  (let [uuid (str (UUID/randomUUID))
+        form (binding [*print-meta* true] (pr-str form))]
+    (boot.App/setRegister uuid form)
+    `(read-string (boot.App/getRegister ~uuid))))
+
 (defn eval-fn-call
   [[f & args]]
   (when-let [ns (namespace f)] (require (symbol ns)))