Add case_exprt to std_expr.h and refactor code to use it #11372

Job	Run time
check-ubuntu-24_04-make-gcc	13m 20s
check-ubuntu-24_04-make-clang	17m 56s
check-ubuntu-24_04-make-clang-smt-z3	15m 42s
windows-msi-package	27m 42s
check-ubuntu-24_04-cmake-gcc-KNOWNBUG	17m 22s
check-vs-2025-cmake-build-and-test	57m 41s
check-ubuntu-24_04-cmake-gcc	30m 57s
check-ubuntu-22_04-cmake-gcc-32bit	27m 25s
check-ubuntu-24_04-cmake-gcc-THOROUGH	22m 7s
check-macos-14-cmake-clang	1h 46m 56s
check-string-table	1m 47s
check-macos-15-intel-make-clang	56m 1s
check-ubuntu-24_04-arm-cmake-gcc	20m 4s
check-vs-2022-make-build-and-test	1h 2m 16s
check-docker-image	20m 7s
check-ubuntu-22_04-make-clang	12m 47s
include-what-you-use	13m 1s
check-ubuntu-24_04-cmake-gcc-14	27m 53s
check-ubuntu-22_04-cmake-gcc	28m 25s
	9h 39m 29s

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