Add peek buffer to io, hide file/buffer private functions

Implements peek buffer on top of the raw io read (with small storage
optimization). Hides implementation details of the file/buffer.

include/bh/internal/file.h

@@ -16,6 +16,38 @@
 int bh_file_init(bh_file_t *file,
                  const char *path);
 
-void bh_file_destroy(bh_file_t *file);
+#define bh_file_destroy(file) \
+    bh_io_destroy((bh_io_t *)(file))
+
+void bh_file_destroy_base(bh_file_t *file);
+
+int bh_file_open_base(bh_file_t *file,
+                      int mode);
+
+void bh_file_close_base(bh_file_t *file);
+
+int bh_file_is_open_base(bh_file_t *file);
+
+size_t bh_file_read_base(bh_file_t *file,
+                         char *data,
+                         size_t size);
+
+size_t bh_file_write_base(bh_file_t *file,
+                          const char *data,
+                          size_t size);
+
+void bh_file_flush_base(bh_file_t *file);
+
+int bh_file_seek_base(bh_file_t *file,
+                      bh_off_t pos,
+                      int dir);
+
+bh_off_t bh_file_size_base(bh_file_t *file);
+
+bh_off_t bh_file_tell_base(bh_file_t *file);
+
+bh_off_t bh_file_available_base(bh_file_t *file);
+
+void bh_file_clear_base(bh_file_t *file);
 
 #endif /* BH_INTERNAL_FILE_H */