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/io.h

@@ -4,10 +4,4 @@
 #include "bh.h"
 #include <bh/io.h>
 
-void bh_io_init(bh_io_t *io,
-                const bh_io_table_t *table);
-
-void bh_io_destroy(bh_io_t *io);
-
 #endif /* BH_INTERNAL_IO_H */
-