YANGTOOLS-766: add RFC7951JSONInstanceIdentifierCodec

YANGTOOLS-766: add RFC7951JSONInstanceIdentifierCodec

RFC7951 non-present prefixes result in last argument namespace reuse,
hence refactor AbstractStringInstanceIdentifierCodec to allow for QNames
to be instantiated using a memoized QNameModule.

The default implementation routes to createQName("", localName), which
preserves backwards compatibility. A specialized subclass
of JSONInstanceIdetifierCodec is created to use memoized QNameModule,
and it is wired to RFC7951 codec supplier.

A similar update is done in AbstractNamespaceCodec, where appendQName()
alternative, which is forwarded the last encountered QNameModule is
added -- the default routes to normal appendQName(), with RFC7951
comparing last QNameModule with current QName's and skipping prefix
serialization if they match.

This is sufficient to make related unit test pass, so remove @Ignore
from it and expand it to assert the decoded YangInstanceIdentifier.

Change-Id: Iec7996d0e1d759623c3cabcbaf0164737afa1649
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>