make "global const" work

[StefanKarpinski]

Tried this:

diff --git a/j/client.j b/j/client.j
index 1e0d5f9..0776a85 100644
--- a/j/client.j
+++ b/j/client.j
@@ -93,21 +93,21 @@ function _start()
     try
         ccall(:jl_register_toplevel_eh, Void, ())
         ccall(:jl_start_io_thread, Void, ())
-        global Workqueue = {}
-        global Waiting = HashTable(64)
+        global const Workqueue = empty(WorkItem)
+        global const Waiting = HashTable(64)

         if !anyp(a->(a=="--worker"), ARGS)
             # start in "head node" mode
-            global Scheduler = Task(()->event_loop(true), 1024*1024)
-            global PGRP = ProcessGroup(1, {LocalProcess()}, {Location("",0)})
+            global const Scheduler = Task(()->event_loop(true), 1024*1024)
+            global const PGRP = ProcessGroup(1, {LocalProcess()}, {Location("",0)})
             # make scheduler aware of current (root) task
             enq_work(roottask_wi)
             yield()
         else
-            global PGRP = ProcessGroup(0, {}, {})
+            global const PGRP = ProcessGroup(0, {}, {})
         end

-        global VARIABLES = {}
+        global const VARIABLES = empty(Symbol)

         # Load customized startup
         try
diff --git a/j/multi.j b/j/multi.j
index a093941..f36e7e1 100644
--- a/j/multi.j
+++ b/j/multi.j
@@ -991,7 +991,7 @@ function start_worker(wrfd)
     # close stdin; workers will not use it
     ccall(dlsym(libc, :close), Int32, (Int32,), int32(0))

-    global Scheduler = current_task()
+    global const Scheduler = current_task()

     worker_sockets = HashTable()
     add_fd_handler(sockfd, fd->accept_handler(fd, worker_sockets))
diff --git a/j/process.j b/j/process.j
index 33ea6b4..db5a0fe 100644
--- a/j/process.j
+++ b/j/process.j
@@ -29,9 +29,9 @@ process_success(s::ProcessExited) = (s.status == 0)

 type FileDes; fd::Int32; end

-global STDIN  = FileDes(ccall(:jl_stdin,  Int32, ()))
-global STDOUT = FileDes(ccall(:jl_stdout, Int32, ()))
-global STDERR = FileDes(ccall(:jl_stderr, Int32, ()))
+global const STDIN = FileDes(ccall(:jl_stdin,  Int32, ()))
+global const STDOUT = FileDes(ccall(:jl_stdout, Int32, ()))
+global const STDERR = FileDes(ccall(:jl_stderr, Int32, ()))

 isequal(fd1::FileDes, fd2::FileDes) = (fd1.fd == fd2.fd)

Doesn't work since global and const don't appear to play nicely together. Not sure all of this diff is correct, but the construct makes sense, at least — it should allow an inner scope to declare a global constant. The opposite order const global should probably also work (or be illegal).