Rename Formatting projects and more

[dougbu]

• rename netstandard Formatting projects
  • Formatting.NetCore and Formatting.NetStandard names no longer make sense
• adjust netstandard Formatting output paths
  • align w/ new project names
• use NETSTANDARD1_3 and Testing_NetStandard1_3
  • NETFX_CORE name no longer makes much sense
  • remove both NETFX_CORE C# defined constant and msbuild property
  • manually-defined constant completely unnecessary in src/ code
• remove RuntimePortable.sln
  • solution name no longer makes much sense and division adds little or no value
  • add .NET Standard projects to Runtime.sln
  • add solution filters for .NET Framework and .NET Standard projects
  • remove $(BuildPortable) property and @(SolutionsToBuild) item group