Add support for fzn_sliding_among constraints. These are now in the m… #75

Job	Run time
build-cmake (windows-latest)	13m 32s
build-autoconf (ubuntu-latest)	8m 27s
build-cmake (macos-latest)	5m 3s
build-autoconf (macos-latest)	3m 42s
build-cmake (ubuntu-latest)	8m 59s
	39m 43s

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