Allow creating a Buffer with no FontSystem

[tigregalis]

Changes

This introduces a new constructor Buffer::new_empty which has zero BufferLines and is unshaped.

Buffer::new calls Buffer::new_empty.

Context

#97 untangled the Buffer from the FontSystem, but didn't remove the need to pass a FontSystem into Buffer::new.

It is useful to be able to initialise a Buffer when you don't have a FontSystem in scope, for example if you want to impl Default for some type that contains a Buffer, since it is otherwise constructable without a FontSystem:

let buffer = Buffer::new_empty(Metrics::new(MY_DEFAULT_FONT_SIZE, MY_DEFAULT_LINE_HEIGHT));

Future work

Any of the examples where Buffer::new is called and then immediately overwritten through Buffer::set_text or overwriting the BufferLines could be rewritten in terms of Buffer::new_empty but I have not done this.

[jackpot51]

This now has conflicts after #122

[tigregalis]

I've resolved the conflict.