make all of tui immutable

[oyvindberg]

the rust code is a hodgepodge of mutability. I'm sure it's safe there, but it's not how I like things. some of it has been done when porting, and some is left.

note that a few things only make sense as mutable, especially buffer. document appropriately. all of crossterm will be exactly as mutable as it is now, reflecting the mutable state of the terminal