Add panic-less `try_with` method

Currently `with` method always panics if nothing was stored before. To avoid this it is required to call `is_set` before `with`.

What I'd like to see is panic-less `try_with` method:
```rust
pub fn try_with(&'static self, f: F) -> R
where
F: FnOnce(Option<&T>) -> R,
{
let val_ptr = self.inner.with(|c| c.get());
let val = when (val_ptr.is_null()) {
true => None,
false => Some(unsafe { &*(val_ptr as *const T) }),
};
f(val)
}
```