Rewrite in Rust

[davidanthoff]

Fixes #87.

Another nice side-effect of this is that we are much closer to a cross-platform solution with this. In fact, juliaup at this point has nothing Windows specific at all in it anymore, I think.

Things still to do:

• Reimplement link.
• Detect x64 vs x86.
• Port launcher.
• Embed correct bundled version in launcher.
• Fix error handling in launcher.
• Remove all download functionality from launcher to make it smaller.
• Rewrite DBGenerator in Rust and put into build.rs.
• Add embedded resource strings like in C++ version.