objfun: Add local header.

This follows pattern already used for objtuple, etc.: objfun.h's content
is not public - each and every piece of code should not have access to it.
It's not private either - with out architecture and implementation language
(C) it doesn't make sense to keep implementation of each object strictly
private and maintain cumbersome accessors. It's "local" - intended to be
used by a small set of "friend" (in C++ terms) objects.

py/objfun.h

@@ -0,0 +1,12 @@
+typedef struct _mp_obj_fun_bc_t {
+    mp_obj_base_t base;
+    mp_obj_dict_t *globals; // the context within which this function was defined
+    machine_uint_t n_args : 15;         // number of arguments this function takes
+    machine_uint_t n_def_args : 15;     // number of default arguments
+    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)
+    mp_obj_t extra_args[];
+} mp_obj_fun_bc_t;