Add "monadic" operations to result

[pdimov]

r & f -> r? f(*r): r.error()
r | x -> r? r: x
r | f -> r? r: f()

https://godbolt.org/z/YWKKbq859

[pdimov]

More like r | x -> r? *r: x, and similarly r | f -> r? *r: f().

[pdimov]

r &= f; -> if( r ) r = f(*r);
r |= x; -> if( !r ) r = x;
r |= f; -> if( !r ) r = f();

[pdimov]

Implemented on develop (without |= which is deemed unnecessary.)

In the future, maybe consider error-transforming variants of |, as in

r | f -> r? *r: f( r.error() )

[pdimov]

r |= x should probably be provided for consistency, always with the effective semantics of r = r | x.

It only makes sense for r |= f; where f returns result, but it's probably less surprising to just support all the cases.

[pdimov]

r |= x implemented on develop.