protocols: Allow them to be (optionally) type-safe

Protocols are nice, but there is no way for C code to verify whether
a type's "protocol" structure actually implements some particular
protocol.  As a result, you can pass an object that implements the
"vfs" protocol to one that expects the "stream" protocol, and the
opposite of awesomeness ensues.

This patch adds an OPTIONAL (but enabled by default) protocol identifier
as the first member of any protocol structure.  This identifier is
simply a unique QSTR chosen by the protocol designer and used by each
protocol implementer.  When checking for protocol support, instead of
just checking whether the object's type has a non-NULL protocol field,
use `mp_proto_get` which implements the protocol check when possible.

The existing protocols are now named:
    protocol_framebuf
    protocol_i2c
    protocol_pin
    protocol_stream
    protocol_spi
    protocol_vfs
(most of these are unused in CP and are just inherited from MP; vfs and
stream are definitely used though)

I did not find any crashing examples, but here's one to give a flavor of what
is improved, using `micropython_coverage`.  Before the change,
the vfs "ioctl" protocol is invoked, and the result is not intelligible
as json (but it could have resulted in a hard fault, potentially):

    >>> import uos, ujson
    >>> u = uos.VfsPosix('/tmp')
    >>> ujson.load(u)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    ValueError: syntax error in JSON

After the change, the vfs object is correctly detected as not supporting
the stream protocol:
    >>> ujson.load(p)
    Traceback (most recent call last):
      File "<stdin>", line 1, in <module>
    OSError: stream operation not supported

1 files changed, 9 insertions, 7 deletions

diff --git a/extmod/modframebuf.c b/extmod/modframebuf.c
index 561b842bc..a51d1c380 100644
--- a/extmod/modframebuf.c
+++ b/extmod/modframebuf.c
@@ -28,6 +28,7 @@
 #include <string.h>
 
 #include "py/runtime.h"
+#include "py/proto.h"
 
 #if MICROPY_PY_FRAMEBUF
 
@@ -46,6 +47,7 @@ typedef uint32_t (*getpixel_t)(const mp_obj_framebuf_t*, int, int);
 typedef void (*fill_rect_t)(const mp_obj_framebuf_t *, int, int, int, int, uint32_t);
 
 typedef struct _mp_framebuf_p_t {
+    MP_PROTOCOL_HEAD
     setpixel_t setpixel;
     getpixel_t getpixel;
     fill_rect_t fill_rect;
@@ -227,13 +229,13 @@ STATIC void gs8_fill_rect(const mp_obj_framebuf_t *fb, int x, int y, int w, int
 }
 
 STATIC mp_framebuf_p_t formats[] = {
-    [FRAMEBUF_MVLSB] = {mvlsb_setpixel, mvlsb_getpixel, mvlsb_fill_rect},
-    [FRAMEBUF_RGB565] = {rgb565_setpixel, rgb565_getpixel, rgb565_fill_rect},
-    [FRAMEBUF_GS2_HMSB] = {gs2_hmsb_setpixel, gs2_hmsb_getpixel, gs2_hmsb_fill_rect},
-    [FRAMEBUF_GS4_HMSB] = {gs4_hmsb_setpixel, gs4_hmsb_getpixel, gs4_hmsb_fill_rect},
-    [FRAMEBUF_GS8] = {gs8_setpixel, gs8_getpixel, gs8_fill_rect},
-    [FRAMEBUF_MHLSB] = {mono_horiz_setpixel, mono_horiz_getpixel, mono_horiz_fill_rect},
-    [FRAMEBUF_MHMSB] = {mono_horiz_setpixel, mono_horiz_getpixel, mono_horiz_fill_rect},
+    [FRAMEBUF_MVLSB] = {MP_PROTO_IMPLEMENT(MP_QSTR_protocol_framebuf) mvlsb_setpixel, mvlsb_getpixel, mvlsb_fill_rect},
+    [FRAMEBUF_RGB565] = {MP_PROTO_IMPLEMENT(MP_QSTR_protocol_framebuf) rgb565_setpixel, rgb565_getpixel, rgb565_fill_rect},
+    [FRAMEBUF_GS2_HMSB] = {MP_PROTO_IMPLEMENT(MP_QSTR_protocol_framebuf) gs2_hmsb_setpixel, gs2_hmsb_getpixel, gs2_hmsb_fill_rect},
+    [FRAMEBUF_GS4_HMSB] = {MP_PROTO_IMPLEMENT(MP_QSTR_protocol_framebuf) gs4_hmsb_setpixel, gs4_hmsb_getpixel, gs4_hmsb_fill_rect},
+    [FRAMEBUF_GS8] = {MP_PROTO_IMPLEMENT(MP_QSTR_protocol_framebuf) gs8_setpixel, gs8_getpixel, gs8_fill_rect},
+    [FRAMEBUF_MHLSB] = {MP_PROTO_IMPLEMENT(MP_QSTR_protocol_framebuf) mono_horiz_setpixel, mono_horiz_getpixel, mono_horiz_fill_rect},
+    [FRAMEBUF_MHMSB] = {MP_PROTO_IMPLEMENT(MP_QSTR_protocol_framebuf) mono_horiz_setpixel, mono_horiz_getpixel, mono_horiz_fill_rect},
 };
 
 static inline void setpixel(const mp_obj_framebuf_t *fb, int x, int y, uint32_t col) {