Moving towards unification of entry point semantics with desktop CLR

[pakrym]

We decided to move towards unification of entry point semantics with desktop CLR.

Breaking changes in RC2:

1. Removed support for async/Task<> Main.
2. Removed support for instantiating of entry point type (Program).
   
   The Main method should be public static void Main or public static int Main.
3. Removed support for injecting dependencies into the Program class's constructor and Main method.
   Use PlatformServices and CompilationServices instead.
   
   To get to IApplicationEnvironment, IRuntimeEnvironment, IAssemblyLoaderContainer, IAssemblyLoadContextAccessor, ILibraryManager
   use Microsoft.Extensions.PlatformAbstractions.PlatformServices.Default static object.
   
   To get to ILibraryExporter, ICompilerOptionsProvider use the Microsoft.Extensions.CompilationAbstractions.CompilationServices.Default static object.
4. Removed support for CallContextServiceLocator. Use PlatformServices and CompilationServices instead.
   
   Same as above.

NOTE: CompilationServices and PlatformServices were added in RC1 so you should start using them to avoid breaking when moving to RC2.