wip: grid is now represented as a grid, not a linear array

The grid is now represented with an array of row *pointers*. Each row
contains an array of cells (the row's columns).

The main point of having row pointers is we can now move rows around
almost for free.

This is useful when scrolling with scroll margins for example, where
we previously had to copy the lines in the margins. Now it's just a
matter of swapping two pointers.

grid.c

@@ -6,7 +6,7 @@
 #define LOG_MODULE "grid"
 #define LOG_ENABLE_DBG 0
 #include "log.h"
-
+#if 0
 struct cell *
 grid_get_range(struct grid *grid, int start, int *length)
 {
@@ -89,3 +89,4 @@ grid_memmove(struct grid *grid, int dst, int src, int length)
         copy_idx += count;
     }
 }
+#endif