Fast reflection operations

The fast reflection operations (relative to the equivalent operations in
FSharp.Core) are the four FSsharpValue.PreCompute___Fast  members for
constructing and reading record types or union types.

I used type augmentation to add the differently named members to Microsoft.Reflection.FSharpValue. Feel free to change that if you don't like it. (I missed @dsyme's last comment in the issue until just now...)

https://github.com/fsharp/fsharpx/issues/234