bufio: make scanning bounded and incremental

lib/regex/regex.ww

@@ -23,6 +23,7 @@ package regex;
 
 import ascii;
 import bufio;
+import errors;
 import io;
 import memio;
 import strconv;
@@ -1209,7 +1210,7 @@ fn search(
 		// Unreachable: this scanner is newscanner(h, I32_MAX), so the
 		// can't-grow ceiling is never hit; the arm keeps the match
 		// total over scanrune's widened overflow surface (drain F-B).
-		case bufio.overflow => abort("regex: scanrune overflow");
+		case errors.overflow => abort("regex: scanrune overflow");
 		};
 		if (r_or_end is rune) {
 			last_bytesize = (utf8.runesz(r_or_end as rune): size);