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, 3 insertions, 1 deletions

diff --git a/py/stream.h b/py/stream.h
index 03e24c746..543fe8c82 100644
--- a/py/stream.h
+++ b/py/stream.h
@@ -27,6 +27,7 @@
 #define MICROPY_INCLUDED_PY_STREAM_H
 
 #include "py/obj.h"
+#include "py/proto.h"
 #include "py/mperrno.h"
 
 #define MP_STREAM_ERROR ((mp_uint_t)-1)
@@ -64,6 +65,7 @@ struct mp_stream_seek_t {
 
 // Stream protocol
 typedef struct _mp_stream_p_t {
+    MP_PROTOCOL_HEAD
     // On error, functions should return MP_STREAM_ERROR and fill in *errcode (values
     // are implementation-dependent, but will be exposed to user, e.g. via exception).
     mp_uint_t (*read)(mp_obj_t obj, void *buf, mp_uint_t size, int *errcode);
@@ -93,7 +95,7 @@ MP_DECLARE_CONST_FUN_OBJ_VAR_BETWEEN(mp_stream_ioctl_obj);
 
 // 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;
+    return mp_proto_get(MP_QSTR_protocol_stream, self); 
 }
 
 const mp_stream_p_t *mp_get_stream_raise(mp_obj_t self_in, int flags);