[exec] cancelable task abstraction

[GuillaumeDua]

See pocs here: https://godbolt.org/z/svarrWzMf

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Transform values of std::invocable<F, ..., stop_token, ...>, Args... into std::invocable<std::stop_token>

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What remains to do ?

Abstraction

• Template constructor instead of template type (as std::function is already used as an underlying type-erasure mechanism)
• conditional operator()
• Better abstraction than std::function -> conditionaly noexcept, const, &/&& call-operator operator().

Execution

• Primitives

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Dependencies

• csl::mp

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Access function -> https://godbolt.org/z/qqo4vr3ed