Moving towards unification of entry point semantics with desktop CLR

[pakrym]

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

Obsolete in RC1:

1. Support for async/Task<> Main.
2. Support for instantiating of entry point type (Program).
   
   The Main method should be public static void Main or public static int Main.
3. 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. Support for CallContextServiceLocator. Use PlatformServices and CompilationServices instead.
   
   Same as above.

These would be removed in RC2: #106