[RFC/WIP] Extract line_consumption.c from misc1.c

This pull request extracts some functions that are concerned with physical line consumption of a buffer on screen from `misc1.c` and puts it in a file called `screenline.c`. (See #209)
@aktau agree, in that case @saimen will be able to remove `#include "nvim/vim.h"` from `screenline.c` too (I think replace it with `"nvim/pos.h"` and `<stdbool.h>` would be sufficient).