bufio: make scanning bounded and incremental

test/wcc/data/r778_scanner_lines/case.ww

@@ -3,6 +3,7 @@
 package main;
 import os;
 import bufio;
+import errors;
 import memio;
 import io;
 export fn main() i32 = {
@@ -19,19 +20,19 @@ export fn main() i32 = {
     case let s: str => { if (s.len == 2 && s[0] == 97u8 && s[1] == 98u8) { ok += 1; }; };
     case io.eof => { return 81; };
     case let e: io.error => { return 82; };
-    case bufio.overflow => { return 83; };
+    case errors.overflow => { return 83; };
     };
     match (bufio.scanline(&sc)) {
     case let s: str => { if (s.len == 1 && s[0] == 99u8) { ok += 1; }; };
     case io.eof => { return 84; };
     case let e: io.error => { return 85; };
-    case bufio.overflow => { return 86; };
+    case errors.overflow => { return 86; };
     };
     match (bufio.scanline(&sc)) {
     case let s: str => { return 87; };
     case io.eof => { ok += 1; };
     case let e: io.error => { return 88; };
-    case bufio.overflow => { return 89; };
+    case errors.overflow => { return 89; };
     };
     bufio.finish(&sc);
     if (ok != 3) { return 90; };