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/buffer.h

@@ -16,6 +16,38 @@ struct bh_buffer_s
 
 int bh_buffer_init(bh_buffer_t *buffer);
 
-void bh_buffer_destroy(bh_buffer_t *buffer);
+#define bh_buffer_destroy(buffer) \
+    bh_io_destroy((bh_io_t *)(buffer))
+
+void bh_buffer_destroy_base(bh_buffer_t *buffer);
+
+int bh_buffer_open_base(bh_buffer_t *buffer,
+                        int mode);
+
+void bh_buffer_close_base(bh_buffer_t *buffer);
+
+int bh_buffer_is_open_base(bh_buffer_t *buffer);
+
+size_t bh_buffer_read_base(bh_buffer_t *buffer,
+                           char *data,
+                           size_t size);
+
+size_t bh_buffer_write_base(bh_buffer_t *buffer,
+                            const char *data,
+                            size_t size);
+
+void bh_buffer_flush_base(bh_buffer_t *buffer);
+
+int bh_buffer_seek_base(bh_buffer_t *buffer,
+                        bh_off_t pos,
+                        int dir);
+
+bh_off_t bh_buffer_size_base(bh_buffer_t *buffer);
+
+bh_off_t bh_buffer_tell_base(bh_buffer_t *buffer);
+
+bh_off_t bh_buffer_available_base(bh_buffer_t *buffer);
+
+void bh_buffer_clear_base(bh_buffer_t *buffer);
 
 #endif /* BH_INTERNAL_BUFFER_H */