Keep a key cut across the terminal's reads until it comes whole

handle_input found escape sequences only whole within one read, so an
arrow whose ESC [ came in one read and its A in the next was Escape, [
and A (Pure's review 004, F5); a UTF-8 character cut the same way went
in pieces. A read's bytes are not the terminal's keys: now an escape
sequence, a mouse report or a character cut short waits for the next
read. An escape alone that nothing follows within 100 ms (vim's
ttimeoutlen) is the Escape key; the rest of a key already begun waits up
to a second.

check.py's interactive runs a third session with every key coming a
byte at a time, 50 ms apart, and the arrow's A 150 ms after its ESC [:
the focus moves, as when keys come whole; Vega before this marks the
wrong task done.

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