extmod/machine_i2c: Change C-level API to allow split I2C transactions.

API is:

    int transfer(
        mp_obj_base_t *obj,
        uint16_t addr,
        size_t n,
        mp_machine_i2c_buf_t *bufs,
        unsigned int flags
    )

extmod/machine_i2c.h

@@ -28,15 +28,24 @@
 
 #include "py/obj.h"
 
+#define MP_MACHINE_I2C_FLAG_READ (0x01) // if not set then it's a write
+#define MP_MACHINE_I2C_FLAG_STOP (0x02)
+
+typedef struct _mp_machine_i2c_buf_t {
+    size_t len;
+    uint8_t *buf;
+} mp_machine_i2c_buf_t;
+
 // I2C protocol
 // the first 4 methods can be NULL, meaning operation is not supported
+// transfer_single only needs to be set if transfer=mp_machine_i2c_transfer_adaptor
 typedef struct _mp_machine_i2c_p_t {
     int (*start)(mp_obj_base_t *obj);
     int (*stop)(mp_obj_base_t *obj);
     int (*read)(mp_obj_base_t *obj, uint8_t *dest, size_t len, bool nack);
     int (*write)(mp_obj_base_t *obj, const uint8_t *src, size_t len);
-    int (*readfrom)(mp_obj_base_t *obj, uint16_t addr, uint8_t *dest, size_t len, bool stop);
-    int (*writeto)(mp_obj_base_t *obj, uint16_t addr, const uint8_t *src, size_t len, bool stop);
+    int (*transfer)(mp_obj_base_t *obj, uint16_t addr, size_t n, mp_machine_i2c_buf_t *bufs, unsigned int flags);
+    int (*transfer_single)(mp_obj_base_t *obj, uint16_t addr, size_t len, uint8_t *buf, unsigned int flags);
 } mp_machine_i2c_p_t;
 
 typedef struct _mp_machine_soft_i2c_obj_t {
@@ -50,7 +59,7 @@ typedef struct _mp_machine_soft_i2c_obj_t {
 extern const mp_obj_type_t machine_i2c_type;
 extern const mp_obj_dict_t mp_machine_soft_i2c_locals_dict;
 
-int mp_machine_soft_i2c_readfrom(mp_obj_base_t *self_in, uint16_t addr, uint8_t *dest, size_t len, bool stop);
-int mp_machine_soft_i2c_writeto(mp_obj_base_t *self_in, uint16_t addr, const uint8_t *src, size_t len, bool stop);
+int mp_machine_i2c_transfer_adaptor(mp_obj_base_t *self, uint16_t addr, size_t n, mp_machine_i2c_buf_t *bufs, unsigned int flags);
+int mp_machine_soft_i2c_transfer(mp_obj_base_t *self, uint16_t addr, size_t n, mp_machine_i2c_buf_t *bufs, unsigned int flags);
 
 #endif // MICROPY_INCLUDED_EXTMOD_MACHINE_I2C_H