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/io.ww

@@ -13,7 +13,7 @@ export type eof = void;
 // closed. NAMED-void; same shape, different tag.
 export type closed = void;
 
-type stream = struct {
+export type stream = struct {
 	ctx:    *void,
 	read:   fn(s: *stream, buf: []u8) (i32 | eof | closed),
 	write:  fn(s: *stream, buf: []u8) (i32 | closed),