This saves 4K in the edit buffers, not to mention all the heap
allocations for the separate mbs buffers!
There might be a way to be more clever about capacities, but I don't
think it's worth it.
Losing tab complete and text macros, for now.
This new implementation works on an instance of a struct and does
not interact with the rest of catgirl, making it possible to copy
into another project. Unlike existing line editing libraries, this
one is entirely abstract and can be rendered externally.
My goal with this library is to be able to implement vi mode. Since
it operates on struct instances rather than globals, it might also
be possible to give catgirl separate line editing buffers for each
window, which would be a nice UX improvement.