Add NormalizationResult

Add NormalizationResult

So far we have had only two possible results: NormalizedNode and
NormalizedMetadata. RFC8528 mount points have hidden in the
NormalizedNode hierarchy -- and we want to change that.

Doing a straighforward extension would end up stepping towards
combinatoric explosion: NormalizedNodeResult and
NormalizedNodeMetadataResult are the effect of two components, one of
which is mandatory. Adding MountPoints would result in four
combinations: data, data+metadata, data+mount, data_metadata+mount.

Rather than going that route, explicitly acknowledge we have multiple
components and concentrate them in NormalizationResult, which at this
point combines NormalizedNode and NormalizedMetadata.

On the receiver side we end up havingNormalizationResultHolder, for the
lack of a more imaginative name.

JIRA: YANGTOOLS-1172
Change-Id: Ib9866fee5223a1eb450e5d40d54d4472bae9adda
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>