* [RELAY] Added an AnnotatedRegionSet utility class

In many of the passes involved in graph partitioning,
we need to extract and manipulate annotated regions.
This class simplifies the extraction of regions from a relay
expression containing region begin and end annotations
as well as providing utility functions to query these
regions and merge them.

Co-authored-by: Ramana Radhakrishnan  <ramana.radhakrishnan@arm.com>

Change-Id: Ia912fea0b99f64b6a7197aa6da2347e58f469fbb

* Rename fix

* Update MakeRegions

* Fix __init__

* Indentation

* Code style

* Remove 'Region' from docs

* Overload [] to get region

* Use src/dest for MergeRegions

* Simplify merge

* Tidy const loop vars