Add method for consuming a Vec and taking one element.

Sometimes I want to take a single element from a Vec and throw the rest of the Vec away. I can do this using swap_remove then dropping the Vec (or letting it fall out of scope) but it would be clearer and slightly more efficient if there was a method for doing this directly.

I propose adding this method to Vec:

impl<T> Vec<T> {
    /// Take a single element of a vector and return it, consuming the
    /// vector. If `index` is invalid the original vector is returned.
    pub fn into_element(self, index: usize) -> Result<T, Vec<T>> {
        ...
    }
}

Start by reviewing the proposed Vec::into_element signature and the six-comment discussion. The payload identifies no implementation files or tests, so first determine the applicable RFC process and affected Rust API area. Done would require resolving the proposal's design and documenting an accepted decision.