Multi-target .NET Standard 2.0 and .NET Framework 4.5

[WilStead]

This PR adds multi-targeting for netstandard2.0 and net45, in order to support use of Migrant in .NET Standard projects.

Summary of changes:

• The sln and csproj files are updated to the VS2017 format to permit the .NET Standard multi-targeting, including moving info from AssemblyInfo.cs to the project files.
• Only one method is not currently supported by .NET Standard 2.0: DumpToLibrary<T> in Generators/GeneratorHelper.cs. This method is unused internally, even by tests. Compiler conditionals are used to restrict its use to the net45 target; for the netstandard2.0 target an Obsolete attribute is added and the method throws a NotImplementedException if called.
• The second module test project had to be moved out of its position nested within the main test project and placed into its own top-level folder, due to a bug in the way the new project format handles automatic assembly info generation.

All tests verified passing after the changes.