py/stream: Introduce and use efficient mp_get_stream to access stream_p.

The existing mp_get_stream_raise() helper does explicit checks that the
input object is a real pointer object, has a non-NULL stream protocol, and
has the desired stream C method (read/write/ioctl).  In most cases it is
not necessary to do these checks because it is guaranteed that the input
object has the stream protocol and desired C methods.  For example, native
objects that use the stream wrappers (eg mp_stream_readinto_obj) in their
locals dict always have the stream protocol (or else they shouldn't have
these wrappers in their locals dict).

This patch introduces an efficient mp_get_stream() which doesn't do any
checks and just extracts the stream protocol struct.  This should be used
in all cases where the argument object is known to be a stream.  The
existing mp_get_stream_raise() should be used primarily to verify that an
object does have the correct stream protocol methods.

All uses of mp_get_stream_raise() in py/stream.c have been converted to use
mp_get_stream() because the argument is guaranteed to be a proper stream
object.

This patch improves efficiency of stream operations and reduces code size.

py/stream.h

@@ -90,6 +90,11 @@ MP_DECLARE_CONST_FUN_OBJ_VAR_BETWEEN(mp_stream_ioctl_obj);
 #define MP_STREAM_OP_WRITE (2)
 #define MP_STREAM_OP_IOCTL (4)
 
+// Object is assumed to have a non-NULL stream protocol with valid r/w/ioctl methods
+static inline const mp_stream_p_t *mp_get_stream(mp_const_obj_t self) {
+    return (const mp_stream_p_t*)((const mp_obj_base_t*)MP_OBJ_TO_PTR(self))->type->protocol;
+}
+
 const mp_stream_p_t *mp_get_stream_raise(mp_obj_t self_in, int flags);
 mp_obj_t mp_stream_close(mp_obj_t stream);