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/700_e2e.c

@@ -1300,35 +1300,49 @@ static const struct row rows[] = {
 	  "fn main() i32 = {\n"
 	  "  return dispatch(classify(1)) + dispatch(classify(2)) + dispatch(classify(3));\n"
 	  "};", 60 },
-	/* bufio.readline: drain successive '\\n'-terminated lines from a
-	 * pre-filled buffer, then a trailing fragment that returns the
-	 * `linerr` variant carrying \"no newline\" (10 chars). */
+	/* bufio.scanline: drain '\\n'-terminated lines from a memio.fixed
+	 * source through a bufio.scanner. The trailing "baz" fragment has
+	 * no newline; under the EOF_DISCARD default it's dropped and the
+	 * call returns io.eof. Also pins the cross-module type ref
+	 * (scanner stores *io.stream) end-to-end through `use`. */
 	{ "use bufio;\n"
+	  "use io;\n"
+	  "use memio;\n"
 	  "fn main() i32 = {\n"
 	  "  let raw: [11]u8;\n"
 	  "  raw[0] = 102u8; raw[1] = 111u8; raw[2] = 111u8; raw[3] = 10u8;\n"
 	  "  raw[4] = 98u8;  raw[5] = 97u8;  raw[6] = 114u8; raw[7] = 10u8;\n"
 	  "  raw[8] = 98u8;  raw[9] = 97u8;  raw[10] = 122u8;\n"
-	  "  let b: bufio.buf;\n"
-	  "  b.s = nil; b.data = raw.ptr; b.cap = 11; b.r = 0; b.w = 11;\n"
+	  "  let mem: memio.state;\n"
+	  "  let m:   io.stream;\n"
+	  "  memio.fixed(&mem, &m, raw[0:11]);\n"
+	  "  let buf: [16]u8;\n"
+	  "  let sc:  bufio.scanner;\n"
+	  "  bufio.newscanner(&sc, &m, buf[0:16]);\n"
 	  "  let acc: i32 = 0;\n"
-	  "  let l1: (str | bufio.linerr) = bufio.readline(&b);\n"
+	  "  let l1: (str | io.eof | io.closed | bufio.overflow) = bufio.scanline(&sc);\n"
 	  "  match (l1) {\n"
-	  "    case let s: str => acc += s.len: i32;\n"
-	  "    case let e: bufio.linerr => acc += -100;\n"
+	  "    case let s: str       => acc += s.len;\n"
+	  "    case io.eof           => acc += -100;\n"
+	  "    case io.closed        => acc += -1000;\n"
+	  "    case bufio.overflow   => acc += -10000;\n"
 	  "  };\n"
-	  "  let l2: (str | bufio.linerr) = bufio.readline(&b);\n"
+	  "  let l2: (str | io.eof | io.closed | bufio.overflow) = bufio.scanline(&sc);\n"
 	  "  match (l2) {\n"
-	  "    case let s: str => acc += s.len: i32;\n"
-	  "    case let e: bufio.linerr => acc += -100;\n"
+	  "    case let s: str       => acc += s.len;\n"
+	  "    case io.eof           => acc += -100;\n"
+	  "    case io.closed        => acc += -1000;\n"
+	  "    case bufio.overflow   => acc += -10000;\n"
 	  "  };\n"
-	  "  let l3: (str | bufio.linerr) = bufio.readline(&b);\n"
+	  "  let l3: (str | io.eof | io.closed | bufio.overflow) = bufio.scanline(&sc);\n"
 	  "  match (l3) {\n"
-	  "    case let s: str => acc += -100;\n"
-	  "    case let e: bufio.linerr => acc += e.len: i32;\n"
+	  "    case let s: str       => acc += -100;\n"
+	  "    case io.eof           => acc += 7;\n"
+	  "    case io.closed        => acc += -1000;\n"
+	  "    case bufio.overflow   => acc += -10000;\n"
 	  "  };\n"
 	  "  return acc;\n"
-	  "};", 16 },	/* 3 + 3 + len(\"no newline\")=10 */
+	  "};", 13 },	/* 3 + 3 + 7 (eof arm) */
 	/* `(scalar, str)` tuple return: AX:DX:CX convention extends the
 	 * tagged-union ABI. AX = scalar, DX = str.ptr, CX = str.len.
 	 * Receive sites destructure off the same regs regardless of