Merge the .Net Native and CoreCLR implementations of EventSource. This replaces a...

Merge the .Net Native and CoreCLR implementations of EventSource. This replaces a lot of the previous ifdefs in the ProjectN impelmentation with a partial class that is cleaner and easier to understand.

Remove some ifdefs that can be done in a better way and merge additional bug fixes that happened in the nuget package branch.

Refactor some code to eliminate ifdefs for projectN.

Refactor code so that the ProjectN support uses partial classes instead of ifdefs.

Round 2 of making sure everything compiles in CoreCLR and in ProjectN.

Add formatting to the debugger message that EventSource prints when an internal error happens.

Change null strings to be represented as empty strings in TraceLogging.

Remove some ifdefs that can be done in a better way and merge additional bug fixes that happened in the nuget package branch.

Change WriteEventString to use an anonymous struct instead of EventMessage.