Edited Issue: Remove miscellaneous source files in src root folder [112]

There are a number of source files in the “src” root folder that seemingly don’t have to be there:

ExceptionHelper.cs: Move to src\Common

HashCodeCombiner.cs: Move to src\Common

VirtualPathUtilityWrapper.cs: Move to src\Common

IVirtualPathUtility.cs: Only referenced by System.Web.WebPages.csproj – move to that project folder

MimeMapping.cs: Only referenced by System.Web.WebPages.csproj – move to that project folder

DynamicHelper.cs: Only referenced by System.Web.Helpers.csproj – move to that project folder