Work well with older lock_api versions

The crate `lock_api` is a transitive dependency, pulled in trough the
use of `parking_lot`. `parking_lot` depends on `lock_api` version
`^0.4.6`, but only since `lock_api` 0.4.7 the `RwLock::new()` method is
available on rust stable, coinciding with the release of rust
[1.61][const_fn_trait_bound].

A problem occurs if you try to use `once_map` with non-up-to-date
dependencies, e.g. by running

```sh
RUSTC_BOOTSTRAP=1 cargo generate-lockfile -Zminimal-versions
```

This PR makes `Shard::new()` non-`const`, so it works with older
`lock_api` versions, too.

src/sync.rs

@@ -181,7 +181,7 @@ struct Shard<K, V> {
 }
 
 impl<K, V> Shard<K, V> {
-    const fn new() -> Self {
+    fn new() -> Self {
         Self {
             map: RwLock::new(HashMap::new()),
             waiters: Mutex::new(HashMap::new()),