Add generalized CanonicalValue YANG concept

Add generalized CanonicalValue YANG concept

The parsing and validation concepts in DerivedString are really
useful in expressing that a particular value has been validated
to conform to some abstract contract.

This facility will allow us to side-step validation when moving
data between classes, especially when we can prove requried
validation has already been done.

This patch introduces CanonicalValue and support interfaces, so
it can be used more widely.

JIRA: YANGTOOLS-418
Change-Id: Ib3f8723facd60c6737351d863ad0f9cef6c4a5da
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>