Statically link Opir with libclang and release binaries

[PMunch]

This would greatly improve the dev story in cases where installing Clang is inconvenient. We should be able to use https://github.com/deech/libclang-static-build to get a fully statically linked version of Opir which can then be shipped as part of the install. This would mean no more libclang runtime dependency, making installing Futhark easier.