xor() combinator is stable since 1.37.0

The combinator pages states:

> Rust nightly support [xor()](https://doc.rust-lang.org/std/option/enum.Option.html#method.xor) for Option types, which returns Some only if one expression got Some, but not both.

And here the relevant part from the std source:

```rust
#[stable(feature = "option_xor", since = "1.37.0")]
pub fn xor(self, optb: Option<T>) -> Option<T>
```