ww: import toolchain — C bootstrap + ww-side self-host (phases 0-10)

C bootstrap (phases 0-9):
  cmd/wwc, cmd/6c, cmd/6a, cmd/6l, cmd/ww, rt, lib/*.

ww-side self-host (phase 10):
  selfhost/cmd/wwc — ww-cgen frontend; bootstrap fixed point.
  selfhost/cmd/6a  — assembler; byte-identical to C 6a (test 991).
  selfhost/cmd/6l  — linker w/ archive (.a) support; byte-identical
                     to C 6l (test 992).
  selfhost/cmd/ww  — driver (build/run/version); byte-identical to
                     C ww (test 993).

make test: 15/15. make bootstrap: ww2.s == ww3.s, ww2.o == ww3.o,
ww2 == ww3 byte-identical, with the full ww-tooled chain.

lib/errors/errors.ww

@@ -0,0 +1,30 @@
+// errors — error type (a string) and a few sentinels. Plan 9 model:
+// the empty string means OK, a non-empty string is the message.
+
+type error = str;
+
+def eEOF:        error = "eof";
+def eShortRead:  error = "short read";
+def eShortWrite: error = "short write";
+def eClosed:     error = "closed";
+def eInvalid:    error = "invalid argument";
+def ePerm:       error = "permission denied";
+def eNotFound:   error = "not found";
+def eExists:     error = "already exists";
+
+export fn isnil(e: error) bool = {
+	return e.len == 0;
+};
+
+// is — compare an error against a sentinel (or any other error). Pure
+// byte equality; same shape as strings.equal but kept here so callers
+// don't have to pull in strings just to compare.
+export fn is(e: error, want: error) bool = {
+	if (e.len != want.len) { return false; };
+	let i: i32 = 0;
+	for (i < e.len) {
+		if (e[i] != want[i]) { return false; };
+		i += 1;
+	};
+	return true;
+};