lib+test: graduate bufio to Hare scanner on io.stream

Scanner subset: newscanner, finish, scanbyte, scanline, scantok,
overflow (named-void). Buffer caller-owned; EOF_DISCARD default.
Skips Hare's scanner-as-io.stream embed pending task #6 (chained
N_DOT-of-N_DOT miscompile).

test/wcc/900_stdlib.c

@@ -33,7 +33,12 @@ static const char *modules[] = {
 	"lib/math/random/random.ww",
 	"lib/time/time.ww",
 	"lib/c/libc/libc.ww",
-	"lib/bufio/bufio.ww",
+	/* lib/bufio/bufio.ww moved off this list: graduates to the
+	 * io.stream-based scanner, which carries cross-module type
+	 * refs (*io.stream, io.eof, io.closed) that only resolve once
+	 * the driver concatenates `use`d modules. Coverage lives at
+	 * lib/bufio/bufiotest.ww + the bufio.scanline e2e row in
+	 * test/wcc/700_e2e.c. */
 	"lib/fmt/fmt.ww",
 	"lib/net/net.ww",
 	NULL