Ship gdb with Visual Rust

[vosen]

Start bundling gdb (both i686 and x86_64) with Visual Rust. Support it from the UI and from the debugging launcher.
There are some caveats, that might require some reconsideration:

• Whole mingw-builds distribution is downloaded at the start of the build (which comes together to something like ~110MB). It's done only once. I'd rather just download only the gdb, but from what I've remember msys2 is rolling release and win-builds doesn't work with Rust (and might be rolling release too, I'm not quite sure).
• We ship with python interface, but I've gutted python distribution to a minimal set of files that don't crash the gdb.exe. This is probably not enought for debug visualizers, but I think we might be required to use VS native visualizers anyway (I've seen .natvis for Android in MIEngine sources). Related to Debug visualization of Rust data structures #160.

[ctaggart]

I was going to blog on how I got I got things setup on Saturday. Adding a gdb was one of the harder parts. Looks like you are eliminating that step. 👍