Fixed a Few Compiler Warnings

[1eyewonder]

Here are a few fixes to some compiler warnings we are getting on builds. I'm wanting to try and go through and attempt to do a few rounds of cleanup to make real issues more obvious when we encounter them.

I'll be making separate PRs for:

1. InstallModel.CreateFromLibs -> InstallModel.CreateFromContent
   1. There are a lot of warnings we can clean up here
2. build.fsx
   1. I think we can eliminate a lot of these warnings by updating our FAKE dependencies and referencing the modern core namespaces instead

[smoothdeveloper]

Can you base your PRs on top of #4281?

It already enables warning as errors, while leaving those that are a bit invasive to fix in the codebase.

[forki]

thanks!