* 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
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