a: buffer: no block storage

[as]

It seems using a byte slice as the underlying storage mechanism for the text worked a bit too well, to the point of where I completely forgot there was no block storage.

Insert/Delete performance begins to degrade at around 128MB on my laptop. At least an in memory block storage should be used for buffers over 4MB-16MB. I believe the traditional Sam editor used something in the realm of 2-4K on disk.