fibers: drop redundant async_void — the variadic async covers nullary workers

async_void :: ufcs (io, worker: Closure() -> $R) -> Future($R) was redundant:
the variadic async :: ufcs (io, worker: Closure(..$args) -> $R, ..$args) binds
$args to the empty pack, so context.io.async(() -> $R => ...) already calls
worker() and returns Future($R). The name was also misleading — it returns
Future($R), not void (a true void form is Future(void), separate, blocked by
issue 0150).

Removed the definition (std/io.sx) + the std.sx re-export; nothing else
referenced it. Locked the nullary path in examples/1805 (prints nullary: 42) so
the coverage async_void provided is not lost. Suite green 736/0.

examples/1805-concurrency-io-blocking-async.sx

@@ -18,6 +18,11 @@ main :: () {
     d := context.io.async((x: i64) -> i64 => x * 2, 21);
     print("double: {}\n", d.await() or { -1 });
 
+    // Nullary worker — the variadic `async` binds an empty pack, so no separate
+    // `async_void` entry is needed.
+    n := context.io.async(() -> i64 => 42);
+    print("nullary: {}\n", n.await() or { -1 });
+
     // The Io capability also carries a monotonic clock.
     if context.io.now_ms() >= 0 { print("clock ok\n"); }
 }

examples/expected/1805-concurrency-io-blocking-async.stdout

@@ -1,3 +1,4 @@
 sum: 42
 double: 42
+nullary: 42
 clock ok