Clean up XML instance identifier class naming

Clean up XML instance identifier class naming

Naming of things here is not great. We have two distinct implementations
of a full Codec, one of which can handle serialization and the other can
handle parsing.

Clean up naming, by having a top-level, fixed,
InstanceIdentifierXmlCodec, which is what we give users. We then have
InstanceIdentifier{Deserializer,Serializer}, which are bound, single-use
delegates.

This allows us to ditch the thread local stack we have been keeping to
deal with the fact that the implementation is directly internal.

Furthermore we optimize instantiation, so that we minimize the number of
calls to DataSchemaContextTree.from().

JIRA: YANGTOOLS-1543
Change-Id: I1ca884fe57086311111f12b3ba223d08f3676a93
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>