Static functions to create error-holding result and value-holding result

[grisumbras]

The constructors that use special tag types from namespace boost::system has the unfortunate effect that one can't write a function template that (reliably) creates a result object without including some Boost.System headers. Thus it creates a physical dependency on System. For example, see this: https://godbolt.org/z/7KsKfreKe.

This can be solved with a pair of static functions result::make_error and result::make_value.

[grisumbras]

BTW, while we are there, having nested type aliases for value type and error type can also be useful.

[pdimov]

It turned out that static functions don't work when the value type is noncopyable, so I've added in_place_value and in_place_error static members to result instead. This allows R( R::in_place_value, args... ) and R( R::in_place_error, args... ).