Add derived statement cache

Add derived statement cache

A single ReactorStmtCtx may have multiple derived
InferredStatementContexts from it, each resulting in a statement
which is different from the statement produced by the original,
while also being the same among themselves.

This happens actually quite often, since typical schema tree
participants record their copy history as well as well as
effectiveConfig, which is different between defining grouping
and instantiation place.

This mechanics is driven via individual implementation opt-ins,
with SchemaTree being the most significant user.

JIRA: YANGTOOLS-1214
Change-Id: Ie3163e5a7fa8c5cfdfb670102deca7cb22b0d935
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>