Updated define constants to address bugzilla 24797 and add missing supported features.

*   Removed FX_NO_STRUCTURAL_EQUALITY, FX_NO_CUSTOMATTRIBUTEDATA as these
are both provided in mscorlib for monotouch/monodroid.
*(FX_NO_STRUCTURAL_EQUALITY actually causes an issue with array
comparisons within tuples, see [bugzilla 24797](https://bugzilla.xamarin.com/show_bug.cgi?id=24797))*

*   Removed FX_NO_BIGINT_CULTURE_PARSE as its already covered by
FX_NO_BIGINT

*   Added FX_ATLEAST_45, FSHARP_CORE_4_5 as .Net 4.5 features defined
within are supported by monotouch/monodroid
(cc @migueldeicaza)

It's really, really important to work this out 100% for MonoDroid and MonoTouch. Let's iterate on this until we are sure we're doing the right thing

Here's what I see

- Currently, the FSharp.Core.dll for MonoDroid and MonoTouch use the _same_ version number. So we are effectively assuming these two platforms are identical w.r.t. FSharp.Core.  We need to work out if that assumption is valid. I think it is.

- Currently, the FSharp.Core for MonoDroid is built against [these DLLs from October 2013](https://github.com/fsharp/fsharp/tree/master/dependencies/mono/2.1), and the FSharp.Core for MonoTouch is built against [these DLLs from 2013 with an extra mscorlib-runtime.dll](https://github.com/fsharp/fsharp/tree/master/dependencies/mono/2.1/MonoTouch).  We should update these DLLs to the latest installed by Xamarin MonoAndroid and MonoTouch SDKs.  Comparing with ``C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\MonoAndroid\v1.0\mscorlib.dll`` these appear out-of-date by now.

- Looking at the differences between FSharp.Core 4.3.1.0 for .NET 4.x and the MonoTouch/MonoDroid, the only differences remaining are FX_NO_BIGINT, FX_NO_EXCEPTIONDISPATCHINFO and FX_NO_CUSTOMATTRIBUTEDATA (the latter for MonoDroid only).  I think _all_ of these can be removed once we update the base DLLs in "dependencies" since ``C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\MonoTouch\v1.0`` and ``C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\MonoAndroid\v1.0`` both appear to contain System.Numerics.dll, a very up-to-date mscorlib.dll with ExceptionDispatchInfo etc.

All in all this makes me wonder why we need a specific build of FSharp.Core for MonoAndroid and MonoTouch at all.  If we can remove the three #defines above, the FSharp.Core will be _identical_ to the .NET 4.x 4.3.1.0 FSharp.Core.  So why don't we just use that instead?  It would make everything much, much simpler to do that.

Thanks
Don