py: Implement default keyword only args.

Should finish addressing issue #524.

py/objfun.h

@@ -30,10 +30,15 @@ typedef struct _mp_obj_fun_bc_t {
     machine_uint_t n_pos_args : 16;     // number of arguments this function takes
     machine_uint_t n_kwonly_args : 16;  // number of arguments this function takes
     machine_uint_t n_def_args : 16;     // number of default arguments
+    machine_uint_t has_def_kw_args : 1; // set if this function has default keyword args
     machine_uint_t takes_var_args : 1;  // set if this function takes variable args
     machine_uint_t takes_kw_args : 1;   // set if this function takes keyword args
     const byte *bytecode;   // bytecode for the function
     qstr *args;             // argument names (needed to resolve positional args passed as keywords)
-    // values of default args (if any), plus a slot at the end for var args and/or kw args (if it takes them)
+    // the following extra_args array is allocated space to take (in order):
+    //  - values of positional default args (if any)
+    //  - a single slot for default kw args dict (if it has them)
+    //  - a single slot for var args tuple (if it takes them)
+    //  - a single slot for kw args dict (if it takes them)
     mp_obj_t extra_args[];
 } mp_obj_fun_bc_t;