sync: Implement From<T> for OnceCell<T>

[sb64]

Motivation

std::lazy::OnceCell implements From<T> for OnceCell<T>, but tokio::sync::OnceCell does not. Generally, tokio tries to match std's API as closely as possible, so this could be considered an API hole.

Solution

This PR adds that implementation. While this is a niche case, it also allows tokio::OnceCell to be constructed from a T. As it is right now, one can construct an uninitialized OnceCell with new and const_new, or one that may or may not be initialized using new_with, but it is not possible to construct one directly from a T (although this could be done with new_with(Some(T))).