Add Windows + MinGW toolchain to the build matrix #164
Description
In the light of recent events (#155) it became clear that it is no longer possible to rely on single Windows environment for CI matrix. Using windows-latest OS and rust: [stable, nightly] in GitHub actions results in tests runing only with default toolchain and target for that platform, which is {channel}-x86_64-pc-windows-msvc. AppVeyor used to test MSYS setup, but currently that is commented out.