This would previously result in an empty line, because the CR resets the
current column to zero. Now we remember the last written char and column
so that we can restore the column before finishing the line.
The line where the cursor located is always displayed at the bottom and
old content scrolls up until disappearing eventually at the top of the
screen.
This is implemented using a ring buffer of all rows, where old chars
are only overwritten when writing new ones. To avoid displaying stale
data at the end of rows, we save the length of every row and hide chars
that were not overwritten (yet).
This adds an explicit terminal_buffer entity to help with the blockram
inference. Both read and write are completely independent, although they
run with the same clock for now.
