lib+test/wcc: add memio, rename io stream.ww→io.ww

Mirrors Hare's memio: fixed, dynamic, dynamicfrom, buffer, string,
reset, borrowedread. Caller owns the state + stream slots because
w6c lacks &x.field and 32B return-by-value. Tests table-driven via
parallel arrays. io.stream now exported.

lib/io/stream.ww

@@ -1,33 +0,0 @@
-// io — stream interface (Plan 9 Bio / Hare io::stream shape).
-//
-// No closures, no methods. A `stream` is a struct of function
-// pointers plus a `ctx: *void`. The error channel is the return
-// value of read/write/close — Hare-shaped tagged unions instead of
-// errno-style integer sentinels.
-
-// eof — read past the end of the stream. NAMED-void so it's a
-// distinct variant tag from `void` (which would be "no result yet").
-export type eof = void;
-
-// closed — operation attempted on a stream that has already been
-// closed. NAMED-void; same shape, different tag.
-export type closed = void;
-
-type stream = struct {
-	ctx:    *void,
-	read:   fn(s: *stream, buf: []u8) (i32 | eof | closed),
-	write:  fn(s: *stream, buf: []u8) (i32 | closed),
-	close:  fn(s: *stream) (void | closed),
-};
-
-export fn read(s: *stream, buf: []u8) (i32 | eof | closed) = {
-	return s.read(s, buf);
-};
-
-export fn write(s: *stream, buf: []u8) (i32 | closed) = {
-	return s.write(s, buf);
-};
-
-export fn close(s: *stream) (void | closed) = {
-	return s.close(s);
-};