SpriteFont Optimizations Pt. 1

This is a first pass at SpriteFont optimization.

My measurements currently show that when rendering a few larger strings MonoGame w/ DirectX 11 is slightly faster than XNA w/ DirectX 9.

This PR improves the performance of MonoGame font rendering by a small amount on the PC.  It is mostly from adding a custom IEqualityComparer for `char` instead of relying on the `GenericEqualityComparer` used by default.  This might prove to be a bigger win under Mono on non-Windows platforms.

While I was at this I did a few small cleanups, but kept them to a minimum.