The `bitcoinconsensus` crate is not fully under our control because it
exposes code from Core, so we cannot guarantee its stability across
versions. To make our semver compliance easier we can fully encapsulate
the `bitcoinconsensus` crate so it does not appear in our public API.
However, it is useful to have the crate itself exported, here we add an
"unstable" feature and only publicly export the `bitcoinconsensus` crate
if the "unstable" feature is enabled.
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