- Elements in the array are now sorted as a linked list.  This makes it easier to
  apply tailorings.
- Added code to sort entries by collation elements.
- Added logical reset points.  This is used for tailoring relative to certain
  properties, rather than characters.

NOTE: all code for type entry should now be in order.go.  To keep the diffs for
this CL reasonable, though, the existing code is left in builder.go.  I'll move
this in a separate CL.

R=r
CC=golang-dev
https://golang.org/cl/6493063