Optimize SubstatementContext size

Optimize SubstatementContext size

Class layout of SubstatementContext contains a set of flags in
StatementContextBase and SubstatementContext, both of which end up
being padded -- wasting 2-10 bytes in the padding alone.

Moving configuration/ignoreConfig/ignoreIfFeature to
StatementContextBase and allocating them as individual bits allows
us to eliminate internal losses in most cases, resulting in net
savings of 8 bytes per instance in the common case (64bit VM), i.e.
4.5-8.3%.

JIRA: YANGTOOLS-652
Change-Id: Ic63f75e9e8c1c25445bc7904c08a5691d1470b67
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>