Make the Environment module fully type safe #17952

Job	Run time
Arch Linux	20m 52s
CUDA (on Arch)	7m 59s
Fedora	20m 21s
Gentoo	25m 33s
OpenSUSE	18m 45s
Ubuntu Bionic	16m 9s
Arch / PyPy	22m 3s
Ubuntu Rolling (gcc, g++)	21m 48s
Ubuntu Rolling (--unity=on -Ddefault_library=static, --no-unittests, gcc, g++)	15m 31s
Ubuntu Rolling (-Ddefault_library=both, --no-unittests, gcc, g++)	17m 6s
Ubuntu Rolling (clang, clang++)	26m 11s
Ubuntu Rolling (--unity=on, --no-unittests, clang, clang++)	18m 15s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, 0, gcc, g++)	15m 12s
Ubuntu Rolling (--cross ubuntu-armhf.json --cross linux-mingw-w64-64bit.json, --unity=on, gcc, g++)	14m 13s
	4h 19m 58s

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