x/tools/container/intsets: add LowerBound

Fixes golang/go#21310.

Change-Id: Id3f23a66b9889a5087c1f83e7d672d14c41a59e3
Reviewed-on: https://go-review.googlesource.com/53432
Reviewed-by: Alan Donovan <adonovan@google.com>
2 files changed