Instead of buffering input in memory one line at a time, we go in "just
in time" mode and always read contents directly from I/O, without
buffering.
It forces us to implement a `ioPutback` scheme, but on the other hand it
greatly simplifies cases where multiple tokens are on the same line
(when a label is directly followed by an instruction).
The end result feels much more solid and less hackish.
This will soon allow use to seek and tell on input, which is necessary
for a second pass which is necessary for forward symbol references.
This require making `blkSel` a bit more flexible. Rather than having one
global selected blkdev, each app can select its own, in its own memory
space.
