Add draft 0.10 release notes
dist: Add libbacktrace to source tarballs
mk: distcheck --uninstall
mk: Workaround distcheck failure on mac. #13224

Mac can't actually build our source tarballs because it's `tar`
command doesn't support the --exclude-vcs flag. This is just
a workaround to make our mac nightlies work (we get our source
tarballs from the linux bot).
auto merge of #13223 : brson/rust/dist, r=alexcrichton
Json doesn't need to depend on IoError to implement Encodable #13230
auto merge of #13230 : Kroisse/rust/encodable-json, r=erickt

The implementation of `Encodable<E, S>` for `Json` doesn't need to assume `S` to be `IoError`, only the default encoders for `Json` are depend on it. So this can be parametrize to increase flexiblity. If not, types that implement `Encoder<E>` where `E` is not `IoError` can't be worked with `Json`.
auto merge of #13189 : alexcrichton/rust/relnotes, r=brson