Add callable gc method exposed to user code, use it in mlist-cycle.rs test (still not quite working; some memory corruption in the recursive tag constructors, not the GC)

src/lib/sys.rs

@@ -3,5 +3,6 @@
   fn size_of[T]() -> uint;
   fn align_of[T]() -> uint;
   fn refcount[T](@T t) -> uint;
+  fn gc();
 }
 

src/rt/rust_builtin.cpp

@@ -81,6 +81,11 @@ refcount(rust_task *task, type_desc *t, size_t *v) {
     return (*v) - 1;
 }
 
+extern "C" CDECL void
+gc(rust_task *task) {
+    task->gc(1);
+}
+
 extern "C" CDECL rust_vec*
 vec_alloc(rust_task *task, type_desc *t, size_t n_elts)
 {

src/test/run-pass/mlist-cycle.rs

@@ -1,5 +1,7 @@
 // -*- rust -*-
 
+use std;
+
 type cell = tup(mutable @list);
 type list = tag(link(@cell), nil());
 
@@ -7,4 +9,6 @@ fn main() {
   let @cell first = tup(@nil());
   let @cell second = tup(@link(first));
   first._0 = link(second);
+  std.sys.rustrt.gc();
+  let @cell third = tup(@nil());
 }