Return a new completion table operating on quoted text.
TABLE operates on the unquoted text.
UNQUOTE is a function that takes a string and returns a new unquoted string.
REQUOTE is a function of 2 args (UPOS QSTR) where
QSTR is a string entered by the user (and hence indicating
the user's preferred form of quoting); and
UPOS is a position within the unquoted form of QSTR.
REQUOTE should return a pair (QPOS . QFUN) such that QPOS is the
position corresponding to UPOS but in QSTR, and QFUN is a function
of one argument (a string) which returns that argument appropriately quoted
for use at QPOS. (fn TABLE UNQUOTE REQUOTE)