The only change from the last release will be the version of librist

librist no longer frees the rist_data_block passed to the callback, so we need
to add a Drop impl to free it on the Rust side

The blocking API actually uses a queue of data from the librist recieve
thread, whereas the callback just runs within the context of the recieve
thread and involve no queue.  This seemed to avoid a bug in librist at
the time this change was made around 5 months ago.

I think this is a bit of a hack, but delivers the result I'm after in
the short term.

output to stderr will be hidden by default

build.rs runs meson unconditionally, but presumably meson is not
available on docs.rs

So just kinda ignore the failure, since presumably linking will just
fail later on in a normal build, and hopefully linking is not attempted
when running 'cargo doc'.