Updates to FsReflect

I have committed an updated version of `FsReflect` to include support for F# union tag readers, tuples and F# exceptions. Corresponding unit tests are included. The public API now includes the following methods: 
* ``FSharpValue.PreComputeExceptionConstructorFast``
* ``FSharpValue.PreComputeExceptionReaderFast``
* ``FSharpValue.PreComputeTupleConstructorFast``
* ``FSharpValue.PreComputeTupleReaderFast`` and 
* ``FSharpValue.PreComputeUnionTagReaderFast``.

The commit incorporates code from the `FsCoreSerializer` project. In my tests I have found performance gains when compared to the original `FsReflect`.