Add `is` and `as` support for `std::expected` (v2) #837

Job	Run time
build-unix-like (ubuntu-22.04, g++-10, c++20)	24s
build-unix-like (ubuntu-22.04, g++-11, c++20)	22s
build-windows	46s
build-unix-like (ubuntu-22.04, g++-11, c++2b)	22s
build-unix-like (ubuntu-22.04, g++-12, c++20)	24s
build-unix-like (ubuntu-22.04, g++-12, c++2b)	27s
build-unix-like (ubuntu-22.04, clang++-12, c++20)	27s
build-unix-like (ubuntu-22.04, clang++-14, c++20)	22s
build-unix-like (macos-latest, clang++, c++20)	14s
build-unix-like (ubuntu-22.04, clang++-15, c++20)	25s
build-unix-like (ubuntu-24.04, clang++-16, c++20)	26s
build-unix-like (ubuntu-24.04, clang++-17, c++20)	57s
build-unix-like (ubuntu-24.04, clang++-18, c++20)	48s
build-unix-like (ubuntu-24.04, clang++-18, c++23)	47s
build-unix-like (ubuntu-24.04, g++-14, c++2b)	22s
	7m 33s

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