manage fiber status (create|running|resumed|terminated)

include/mruby.h

@@ -61,6 +61,13 @@ typedef struct {
   struct REnv *env;
 } mrb_callinfo;
 
+enum mrb_fiber_state {
+  MRB_FIBER_CREATED = 0,
+  MRB_FIBER_RUNNING,
+  MRB_FIBER_RESUMED,
+  MRB_FIBER_TERMINATED,
+};
+
 struct mrb_context {
   struct mrb_context *prev;
 
@@ -75,6 +82,7 @@ struct mrb_context {
   struct RProc **ensure;
   int esize;
 
+  uint8_t status;
   struct RFiber *fib;
 };