Improve ListEntryNodeDataWithSchema

Improve ListEntryNodeDataWithSchema

ImmutableMapTemplate allows us to more efficiently instantiate
NodeIdentifierPredicates, as it takes care of ordering of keys
while maintaining an efficient internal representations.

It furthermore provides a keySet() method, which is useful for
quickly checking if a leaf is part of key definition -- unlike
ListSchemaNode.getKeyDefinition(), which is a List and therefore
must be linearly searched.

It also allows us to skip HashMap allocation for unkeyed lists,
clearly showing that the class should be split into two.

JIRA: YANGTOOLS-917
Change-Id: If2d7bbbc5c4fe1ca96097f374f4313aabb411908
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>