Add AugmentationIdentifier.create() methods
This adds two flavors of AugmentationIdentifier.create(), which
use a global weak cache for normalizing AugmentationIdentifiers.
The cache is carefully populated, i.e. it makes a point of using
immutable keys only.
JIRA: YANGTOOLS-998
Change-Id: I1e7d65b995e50773ec2260c9fb01bc1c6bbaafbf
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit
700dcb02f55c7172c61f16a8318901b276238242)
(cherry picked from commit
0cc211c923a4f925a73fc598baf9d2fe4adf3216)