Fix memory managment around the FFI-barrier #4985

Job	Run time
check-rust	3m 47s
scarb-metadata compatibility check	42s
check-website	18s
build test linux x86-64	2m 54s
build test windows x86-64	3m 29s
test linux x86-64 1/8	3m 46s
test linux x86-64 2/8	3m 58s
test linux x86-64 3/8	4m 0s
test linux x86-64 4/8	3m 18s
test linux x86-64 5/8	3m 40s
test linux x86-64 6/8	1m 46s
test linux x86-64 7/8	2m 14s
test linux x86-64 8/8	2m 11s
test windows x86-64 1/8	3m 48s
test windows x86-64 2/8	3m 58s
test windows x86-64 3/8	4m 19s
test windows x86-64 4/8	3m 29s
test windows x86-64 5/8	3m 24s
test windows x86-64 6/8	2m 2s
test windows x86-64 7/8	2m 34s
test windows x86-64 8/8	2m 33s
	1h 2m 10s

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