Scan from character number FROM by COUNT balanced expressions.
If COUNT is negative, scan backwards.
Returns the character number of the position thus found.
Comments are ignored if `parse-sexp-ignore-comments' is non-nil.
If the beginning or end of (the accessible part of) the buffer is reached
in the middle of a parenthetical grouping, an error is signaled.
If the beginning or end is reached between groupings
but before count is used up, nil is returned.
(fn FROM COUNT)