Move Environment to shared CoreLib (dotnet/corefx#34654)

Move Environment to shared CoreLib (dotnet/corefx#34654)

Rather than having Environment partially live in corefx and call into an EnvironmentAugments type in CoreLib that in turn calls into an Environment type in CoreLib, we're just moving Environment to live in CoreLib.

To start that, this PR moves Environment and its dependencies from their current locations into the shared CoreLib.  Those changes will mirror over to coreclr.  After that, I'll fix it up to work in CoreLib. And once those changes are built and available back to corefx, I'll update System.Runtime.Extensions to just use the functionality from CoreLib and delete remaining unnecessary code from corefx.

Commit migrated from https://github.com/dotnet/corefx/commit/b2d890a311ff66857339c7dae493efa43237284c