Add some more extensions from Lua 5.2/5.3.

Contributed by François Perrad.

src/lib_base.c

@@ -23,6 +23,7 @@
 #include "lj_tab.h"
 #include "lj_meta.h"
 #include "lj_state.h"
+#include "lj_frame.h"
 #if LJ_HASFFI
 #include "lj_ctype.h"
 #include "lj_cconv.h"
@@ -557,6 +558,12 @@ LJLIB_CF(coroutine_running)
 #endif
 }
 
+LJLIB_CF(coroutine_isyieldable)
+{
+  setboolV(L->top++, cframe_canyield(L->cframe));
+  return 1;
+}
+
 LJLIB_CF(coroutine_create)
 {
   lua_State *L1;