Interpreter I/O

The INTERPRET loop, the heart of Collapse OS, feeds itself
from the C< word, which yields a character every time it is
called. If no character is available to interpret, it blocks.

During normal operations, C< is simply a buffered layer over
KEY, which has the same behavior (but unbuffered). Before
yielding any character, the C< routine fetches a whole line
from KEY, puts it in a buffer, then yields the buffered line,
one character at a time.

Both C< and KEY can be overridden by setting an alternate
routine at the proper RAM offset (see B80). For example, C<
overrides are used during LOAD so that input comes from
disk blocks instead of keyboard.                        (cont.)
