Separate out AbstractNormalizedNodeDataInput

Separate out AbstractNormalizedNodeDataInput

As we are going to introduce a streaming format for Magnesium, which
uses very different token set, it is convenient to have a common
base class which does not have conotations about what the tokens are.

We provide a baseline counterpart to AbstractNormalizedNodeDataOutput
and base AbstractLithiumDataInput on it. The asymmetry of
writeYangInstanceIdentifier() is fixed by moving it to
AbstractLithiumDataOutput.

Change-Id: I55d318349f2c89508f3834c8ca5b69e69b7171b1
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>