Skip over one sexp.
HALFSEXP if non-nil, means skip over a partial sexp if needed. I.e. if the
first token we see is an operator, skip over its left-hand-side argument.
HALFSEXP can also be a token, in which case we should skip the text
assuming it is the left-hand-side argument of that token.
Possible return values:
(LEFT-LEVEL POS TOKEN): we couldn't skip TOKEN because its right-level
is too high. LEFT-LEVEL is the left-level of TOKEN,
POS is its start position in the buffer.
(t POS TOKEN): same thing but for an open-paren or the beginning of buffer.
Instead of t, the `car' can also be some other non-nil non-number value.
(nil POS TOKEN): we skipped over a paren-like pair.
nil: we skipped over an identifier, matched parentheses, ...
(fn &optional HALFSEXP)