Changes to constant handling - faster deduplication, more compact represtntation #1361

Job	Run time
build (--test-libcore)	46s
build (--test-successful-rustc --nb-parts 2 --current-part 0)	47s
build (--std-tests)	48s
build (--mini-tests)	47s
build (--test-successful-rustc --nb-parts 2 --current-part 1)	44s
success_gcc12	2s
	3m 54s

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