Refactor KeyEffectiveStatementImpl

Refactor KeyEffectiveStatementImpl

KeyEffectiveStatementImpl costs 32 bytes each instance in common
Linux deployment. This turns out to be unnecessary, as be can easily
drop the size to 16/24 bytes, based on whether the key is referenced
in the same module as it is defined.

The method we do this by is to provide 4 distinct implementations,
based on 2x2 matrix of locality w.r.t declared statement (same module
means we can can reuse declared argument) and number of substatements
-- "key" with a substatement is exceedingly unlikely, hence we can
usually not store substatements.

JIRA: YANGTOOLS-1065
Change-Id: Id1235131a8568236870521cb988e730a233ea96c
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>