Implement deduction guides

[wak-google]

@Naios

It would be nice if function_base supported deduction guides similar to std::function so that lambdas can have sensible type erased deductions.

https://en.cppreference.com/w/cpp/utility/functional/function/deduction_guides

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Commit Hash

2d3a878ef19dd5d2fb188898513610fac0a48621

Expected Behavior

This should compile

fu2::unique_function([](int &i) { return i + 5; })

[Naios]

The idea is awesome, I would highly appreciate a pullrequest for this feature.