lib: graduate bytes/strings find-funcs to (i32 | void)

Replaces the -1 sentinel return on indexbyte/byteindex/rbyteindex/
index with Hare's optional-shaped tagged union. Callers `match` on
the result and bind the index from the i32 variant.

Two cgen fixes were needed first:

1. resolve_type for N_TTAGGED rounded value payload up to an 8-byte
   multiple. (i32 | void) was sized 12 — tag (8) + payload (4) —
   which made the reg-passing ABI compute size/8 = 1 word and drop
   the value word.

2. The call-arg push path special-cased struct and slice args but
   not tagged-return calls. A nested `f(g())` where g returns a
   tagged union pushed only AX (tag); the matching pop loaded a
   stale DX/SI for the value. Now pushes AX/DX[/CX] in order so
   the pop side drains tag → arg-reg[0], value(s) → arg-reg[1..].

strings.contains rewritten to match on the new tagged result. No
other callers existed in lib/ — bufio/io still use their own
shapes.

cmd/wcc/check.c

@@ -192,7 +192,12 @@ resolve_type(Checker *c, Node *n)
 		if (nv == 0) return ty_never;
 		if (nv == 1 && head) return head->type;
 		t->params = head;
-		t->size = 8 + maxsz;
+		/* Round value payload up to an 8-byte multiple so the slot
+		 * layout (tag + N value words) stays word-aligned. The reg-
+		 * passing ABI counts size/8 words; 12-byte unions like
+		 * (i32 | void) would otherwise lose a value register. */
+		u64 vsz = (maxsz + 7) & ~(u64)7;
+		t->size = 8 + vsz;
 		t->align = al;
 		return t;
 	}