Fix ImmutableLeaf(SetEntry)Node.toString()
Allow the class identity of AbstraectNormalizedNode subclass to be
overridden, so that we can have externally-invisible class hierarchy.
Since we want to assert stable toString(), this patch also addresses
NodeWithValue.toString().
In both cases byte[] values are encoded with 'b64:' prefix followed by
base-64 encoded value.
JIRA: YANGTOOLS-1562
JIRA: YANGTOOLS-1563
Change-Id: I686eeb4769d9ebdcf21f7c11b314ccc85fe943fb
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>