Add support for compiling with Visual Studio 2017

[lewissbaker]

The Cake build system currently relies on the registry to determine whether a particular Visual Studio version is installed and if so where it is installed.

Once lewissbaker/cake#8 has been addressed in Cake then we need to update the config.cake for cppcoro to detect whether VS 2017 is available and if so then configure a new MsvcCompiler tool that makes use of it.

[lewissbaker]

I could also potentially try and pull the compiler from http://vcppdogfooding.azurewebsites.net/.

This would actually be somewhat simpler as once downloaded you can just unzip the .nupkg to a known location and use the compiler directly from there. You just can't use this approach for anything production-related. Nevertheless it may well be useful for testing out updates to coroutines in pre-release versions of Visual Studio.

[lewissbaker]

I've added some initial work towards building with VS2017 in 0f6a170

It just looks in common locations for the install for now but should work for most people until I figure out how to call the COM api for enumerating install locations.

[lewissbaker]

Cake has now gained ability to find VS 2017 using the vswhere utility in lewissbaker/cake@ccb64fd

Commit af6f2fb updates cppcoro to use this functionality from cake.