Move FormatDouble/Single to managed code

Move FormatDouble/Single to managed code

Instead of making fcalls to FormatDouble and FormatSingle, move them to managed, and use fcalls for the DoubleToNumber and NumberToDouble they call, shifting down the layer that's implemented in native.  This allows us to then much more easily add TryFormat methods for double and float while also sharing more code between coreclr and corert, from which the managed implementations were taken (they're a direct port of these native implementations from coreclr).  In the process, I also eliminated one fcall that can be implemented in managed easily.  The remaining fcalls are more substantial and will eventually require more effort to bring to managed.