extmod/vfs: Introduce a C-level VFS protocol, with fast import_stat.

Following other C-level protocols, this VFS protocol is added to help
abstract away implementation details of the underlying VFS in an efficient
way.  As a starting point, the import_stat function is put into this
protocol so that the VFS sub-system does not need to know about every VFS
implementation in order to do an efficient stat for importing files.

In the future it might be worth adding other functions to this protocol.

extmod/vfs.h

@@ -45,6 +45,11 @@
 #define BP_IOCTL_SEC_COUNT      (4)
 #define BP_IOCTL_SEC_SIZE       (5)
 
+// At the moment the VFS protocol just has import_stat, but could be extended to other methods
+typedef struct _mp_vfs_proto_t {
+    mp_import_stat_t (*import_stat)(void *self, const char *path);
+} mp_vfs_proto_t;
+
 typedef struct _mp_vfs_mount_t {
     const char *str; // mount point with leading /
     size_t len;