core: Export future mod

Author: brson

src/libcore/core.rc

@@ -33,7 +33,8 @@ export uint, u8, u16, u32, u64;
 export float, f32, f64;
 export box, char, str, ptr, vec, bool;
 export either, option, result, iter;
-export ctypes, sys, unsafe, comm, task, logging;
+export ctypes, sys, unsafe, logging;
+export comm, task, future;
 export extfmt;
 export math, bessel;
 export tuple;