(line-move-invisible): New subroutine.

(line-move-invisible): New subroutine.
(line-move-to-column): New subroutine--smarter about advancing over
invisible parts of a line, or lines, but only as long as hpos grows.
(line-move-finish): New subroutine: repeatedly processes desired
column, intangibility, and fields.
(line-move): Use those subroutines.
When moving lines downward, skip invisible text first rather than last.