Refactor ListEffectiveStatementImpl

Refactor ListEffectiveStatementImpl

ListEffectiveStatement implementation is immensely big -- it takes
anywhere between 112 and 224 bytes (120 typical), most of which are
really only indices on substatements, which we can construct on demand.

Applying mixin + layout classes here allows us to drop the size of
these statements to 40/48 bytes typical, with 88 bytes worst case.

The effect on retained memory is ~12% memory foot print reduction,
driven by both reduction of number of ImmutableSets and reduced object
size.

JIRA: YANGTOOLS-1065
Change-Id: Iebcf16b500a874dff94bb21b6f9f59bf056fea51
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>