Enhanced Framebuffer impl: Added Textbuffer and implemented fmt::Write

[jounathaen]

Still very slow and glitchy output. I guess that every pixel write causes a VM exit or similar.

[jounathaen]

Probably still not the best approach. What I've seen, what other bootloaders do, is:

• Use GOP Blts for data transfer to the framebuffer.
• Implement the scrolling with ptr copy instead of having a write buffer.

The Redox Bootloader seems to be a good reference in that regard.

[sarahspberrypi]

Looks great! I'll look into the Redox Bootloader when I get the chance :-)