Make spawn accept lvalue scope tokens

[gamecentric]

Spawn only accepts rvalues scope tokens, which is unexpected, since tokens are typically value-oriented, cheaply copiable, objects:

counting_scope scope;
spawn(just(), scope.get_token()); // ok

auto token = scope.get_token();
spawn(just(), token); // error: counting_scope::token& does not satisfy the scope_token concept
spawn(just(), std::move(token)); // ok

See issue #1963

[copy-pr-bot]

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