Add lifetimebound attribute

[patrickroberts]

Here's the motivating case:

any_view<int> foo() {
    vector<int> vec;
    // return {vec};  // not ok, reference dangles
    return vec;  // ok, NRVO moves
}

Getting this return statement correct is subtle and error-prone, and the lifetimebound attribute helps catch dangling references as compilation errors.

[coveralls]

coverage: 81.25%. remained the same — lifetimebound into main