bufio: make scanning bounded and incremental

lib/bufio/stream_test.ww

@@ -5,6 +5,7 @@
 package bufio_test;
 
 import bufio;
+import errors;
 import io;
 import memio;
 
@@ -415,7 +416,7 @@ fn checkpartialflush(zero: bool) void = {
 	let sbuf: [32]u8;
 	let sc:   bufio.scanner = bufio.newscannerbuf(p, sbuf[0:32]);
 
-	let lr: (str | io.eof | io.error | bufio.overflow) = bufio.scanline(&sc);
+	let lr: (str | io.eof | io.error | errors.overflow) = bufio.scanline(&sc);
 	match (lr) {
 	case let v: str => {
 		assert(!(v.len != 8));	// "XYZhello"
@@ -424,7 +425,7 @@ fn checkpartialflush(zero: bool) void = {
 	};
 	case io.eof => abort();
 	case let e: io.error => abort();
-	case bufio.overflow => abort();
+	case errors.overflow => abort();
 	};
 	bufio.finish(&sc);
 };