Implement gap tracking in the segment set.

This change was derived from a change by:
  Reapor-Yurnero <reapor.yurnero@gmail.com>

And has been modified by:
  Adin Scannell <ascannell@google.com>

(The original change author is preserved for the commit.)

This change implements gap tracking in the segment set by adding additional
information in each node, and using that information to speed up gap finding
from a linear scan to a O(log(n)) walk of the tree.

This gap tracking is optional, and will default to off except for segment
instances that set gapTracking equal to 1 in their const lists.

PiperOrigin-RevId: 312621607

pkg/segment/BUILD

@@ -21,6 +21,8 @@ go_template(
     ],
     opt_consts = [
         "minDegree",
+        # trackGaps must either be 0 or 1.
+        "trackGaps",
     ],
     types = [
         "Key",