Make rist_data_block reference counted & add free function.

Unfortunately this is really necessary so late in the release process. The previous design being able to free data from under a calling application.
5 jobs for v0.2.0-RC3 in 1 minute and 4 seconds (queued for 1 second)
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  Build
passed #522838
macos
build-macos

00:00:19

passed #522836
amd64 avx2 docker
build-ubuntu

00:00:12

passed #522837
amd64 docker
build-win64

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  Analyze
passed #522839
amd64 docker
analyze-ubuntu

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  Test
passed #522840
amd64 docker
test-ubuntu

00:00:48