Add support for swift

[pavelzw]

It would be cool if we had the swift compiler available on conda-forge.
Would this be possible license-wise?

cc @wolfv @danielnachun

[danielnachun]

It has the Apache-2.0 license, so yes this should be possible provided we build it from source. I made an attempt at locally building the compiler a while ago and got pretty far. It doesn't require any new dependencies and seems to be a fairly standard CMake build.

Like with our other compiler tool chains, we should ultimately aim to have a compiler('swift') jinja available. Cross-compilation seems like it's now mostly available as described in https://swiftinit.org/articles/cross-compiling-x86_64-linux-to-aarch64-linux.

The only thing I'm not too sure about yet is license bundling for third party dependencies - https://github.com/cybozu/LicenseList seems like it might be what we want and should probably be the first package we build with it. I mention this because static linking does seem to be the default for many Swift packages, though if we can enforce dynamic linking that would be much preferred.