rt: Set the default stack size to 768 bytes. Double on each alloc

src/rt/rust_env.cpp

@@ -67,9 +67,6 @@ get_num_threads()
     return get_num_cpus();
 }
 
-// FIXME (issue #151): This should be 0x300; the change here is for
-// practicality's sake until stack growth is working.
-
 static size_t
 get_min_stk_size() {
     char *stack_size = getenv(RUST_MIN_STACK);
@@ -77,7 +74,7 @@ get_min_stk_size() {
         return strtol(stack_size, NULL, 0);
     }
     else {
-        return 0x300000;
+        return 0x300;
     }
 }