Improves handling `size_t` FFI values. #512

Job	Run time
Check minimum supported rust version (MSRV)	9m 41s
Check documentation	3m 41s
Check spelling	9s
Check formatting	7s
Fedora tests	8m 18s
Fedora rawhide tests	1m 38s
Ubuntu tests	3m 9s
Ubuntu tests on v3.x.y of tpm2-tss	5m 17s
Valgrind test run	4m 31s
	36m 31s

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