Allow implicit conversion to T to be able to use as optional<T>.value_or(...) parameter

[slnj]

I'd like to be able to use Lazy as parameter for optional.value_or and make it be evaluated only if it is required.

Since the parameter must be implict convertible to T ( must verify is_convertible trait ), it should be good if
explicit operator reference() const;
is changed to
operator reference() const;

This will allow to do things like:

auto getFromFile = []() { std::cout << "File opened:   "; return std::make_tuple("Lazy get the value from some file!");};
auto lazy_get_from_file  = lazy::Lazy<std::string>( getFromFile );

std::optional<std::string> a= "Code value";
std::optional<std::string> b;

std::cout << "::Not empty optional::  "<< a.value_or( lazy_get_from_file ) << std::endl;
std::cout << "::Empty optional::  " << b.value_or( lazy_get_from_file ) << std::endl;

and get this as output:

::Not empty optional::  Code value
::Empty optional::  File opened:   Lazy get the value from some file!

Also bool converter should be removed and implemented as a member function like is_instantiated() in cast T is bool
Thanks