Add `is` and `as` support for `std::expected` #55

Job	Run time
Run on ubuntu-latest using g++-10	2m 49s
Run on ubuntu-latest using g++-13	3m 6s
Run on ubuntu-20.04 using clang++-12	2m 48s
Run on macos-13 using clang++	3m 14s
Run on windows-latest using cl.exe	2m 36s
	14m 33s

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