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/sema.nim

@@ -1010,8 +1010,8 @@ proc checkExpr(sema: var Sema, expr: Expr, scope: Scope): Type =
     return makePointer(makeVoid())
   of ekAwait:
     let operand = sema.checkExpr(expr.exprAwaitOperand, scope)
-    # await on a task handle returns void for now
-    return makeVoid()
+    # await on a task handle returns *void (result pointer)
+    return makePointer(makeVoid())
   of ekSpread:
     return sema.checkExpr(expr.exprSpreadOperand, scope)