Shorten switchOnType, function_traits and connect*

...thanks to C++20 awesomeness. A notable change is that
wrap_in_function() (and respectively function_traits<>::function_type)
and fn_return_t alias are gone. The former are no more needed because
connectUntil/connectSingleShot no more use std::function. The latter
has been relatively underused and with the optimisation of switchOnType
hereby, could be completely replaced with std::invoke_result_t.

Rewriting connect* functions using constexpr and auto parameters made
the implementation 30% more compact and much easier to understand
(though still with a couple of - now thoroughly commented - tricky
places). Dropping std::function<> from it may also bring some (quite
modest, likely) performance benefits.

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