Split H.D.EF into a new Repository

[trayburn]

After considerable investigation, it unfortunately becomes clear that Highway.Data.EntityFramework simply will not compiled on the Mono runtime currently. As such my proposal is that we split this repository as follows:

• Highway.Data - All projects that will compile, test, and otherwise successfully work on both OSX/Linux and Windows.
• Highway.Data.EntityFramework - The H.D.EF codebase

Any future projects that would be platform specific, if they exist, would new individual repositories of their own. I am specifically trying to create as few repositories as we can, as I am not interested in the "one repo per package" model of NPM.

[trayburn]

Dropping this Issue given how Mono and .NET Core have developed over the last year.