Changes to constant handling - faster deduplication, more compact represtntation #886

Job	Run time
build (libgccjit.so, master)	7m 12s
build (libgccjit_without_int128.so, master-without-128bit-integers)	7m 0s
build (libgccjit12.so, gcc12, --no-default-features, TEST_FLAGS='-Cpanic=abort -Zpanic-abort-test...	1m 16s
success_failures	2s
	15m 30s

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