feat: async functions with return values

- runtime: bux_async_return(value, size) copies result to task->result
- runtime: bux_async_await returns void* result pointer
- runtime: bux_async_result(handle) to retrieve result
- sema: ekAwait returns *void
- HIR: await lowered to bux_async_await returning *void
- Example: async Compute() -> int with interleaved execution and result await

src/hir_lower.nim

@@ -770,7 +770,7 @@ proc lowerExpr(ctx: var LowerCtx, expr: Expr): HirNode =
 
   of ekAwait:
     let lowered = ctx.lowerExpr(expr.exprAwaitOperand)
-    return hirCall("bux_async_await", @[lowered], makeVoid(), loc)
+    return hirCall("bux_async_await", @[lowered], makePointer(makeVoid()), loc)
 
   else:
     return HirNode(kind: hLit, litToken: Token(kind: tkIntLiteral, text: "0", loc: loc),