Moved the substring tests into a new Substring.fs file to mirror the recent change in ExtCore.
Fix the ExtCore project file so it includes the new Substring.fs.
Fixed a fatal bug in substring.GetSlice() where the length value of the "sliced" substring wasn't calculated correctly and caused an exception to be raised.
Implemented substring-based versions of the trim/trimWith functions from the String module.
Modified the trimWith functions in the String module to call the substring-based versions; the trim functions are left as-is because they're just aliases for optimized methods provided by System.String.