Configuring the runtime has essentially three common cases:

1. .NET Framework
2. Mono with a globally accessiblelibmono.so
3. .NET Core

To make this easier to use (and hideclr_loader a bit), I propose that we introduce a new environment variablePYTHONNET_RUNTIME that configures what happens if one just importsclr, and a correspondingpythonnet.configure function with some simple parameters.