Retain grouping/uses instantiation vectors

Retain grouping/uses instantiation vectors

We need the ability fo find all instantiations of a grouping for
closed-world analysis of BindingRuntimeTypes.

This analysis is need to determine, for example:
- possible types of 'type leafref's pointing outside a grouping,
  to determine which Binding/DOM codecs are applicable
- YANG/Binding overload mapping, i.e. whether a 'container'
  defined in a grouping is instantiated only once or multiple
  types, to use a strongly-bound CodecDataObject it the former case

This patch exposes a GroupingRuntimeType.instantiations(), which exposes
exactly this information.

DefaultGroupingRuntimeType stores this information in the form of
a set of vectors, each pointing either to another grouping or to a
concrete instantiation.

GeneratorReactor collects this information sufficiently early so that it
can be also used to perform partial closed-world analysis during
compile-time.

JIRA: MDSAL-669
Change-Id: I2e21a6b93ce30d9bd1022be5747d44663b6198fc
Signed-off-by: Samuel Schneider <samuel.schneider@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>