Make yang.common base types non-final

Make yang.common base types non-final

While final classes are useful, we will require the ability to mark
validation level of a particular piece of data in a vein similar to
DerivedString -- which requires subclassing to work memory-efficiently.

This patch prepares for that work by making the types non-final,
but does not allow their particulars to be overridden, so they still
form an effectively-final contract just like DerivedString
representations do.

Change-Id: Id7bde4b49d8b8aeb87c9b6e7317e5aa8f55e98eb
JIRA: YANGTOOLS-418
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>