Add filename and SHA256 hash for Windows Rust package
When running `make download` in the `depends` directory, the `download-win` target (which `download` depends on) generates an error when it runs the commands defined in `check_or_remove_sources`:
```Makefile
sha256sum: /home/vagrant/zcash/depends/work/download/rust-1.16.0/..hash: no properly formatted SHA256 checksum lines found
funcs.mk:242: recipe for target '/home/vagrant/gitian-builder/cache/common/download-stamps/.stamp_fetched-rust-.hash' failed
make[1]: *** [/home/vagrant/gitian-builder/cache/common/download-stamps/.stamp_fetched-rust-.hash] Error 1
make[1]: Leaving directory '/home/vagrant/zcash/depends'
Makefile:153: recipe for target 'download-win' failed
make: *** [download-win] Error 2
```
The reason for the error is that `depends/packages/rust.mk` defines `rust_file_name_linux` and `rust_file_name_darwin` but leaves `rust_file_name_mingw32` undefined.
A directory of available rust downloads is here: https://static.rust-lang.org/dist/index.html
The closest windows analog in that list (using the same version number as currently defined in `rust.mk`) appears to be `rust-mingw-1.16.0-x86_64-pc-windows-gnu.tar.gz`. A corresponding sha256 value is also given in `rust-mingw-1.16.0-x86_64-pc-windows-gnu.tar.gz.sha256`.
After adding these values to `rust.mk`, the rust-mingw tar package was downloaded along with the rest of the dependencies and the above error message went away.
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