Major rewrite of text property mechanism. New implementation uses

Major rewrite of text property mechanism.  New implementation uses
red/black trees for properties within intervals, and uses red/black
balancing algorithm for intervals.  Interface to editor is simplified,
as is the interface to the undo mechanism.  The redisplay code no
longer uses the internals of the implementation, but instead uses
standard external entries.  Adjacent intervals are now merged together
when their property sets are the same; property data are compared for
equality using EQV?.