Migrate AbstractTypeStatementSupport
Support for 'type' statement is rather twisted, but at the end of
the day, externalizing substatement creation is rather easy.
Switching to BaseStatementSupport allows us to discern when we can
end up pointing to a BuiltinTypeStatement -- since there is not such
thing as an implicit type statement.
On the declared front, this is a straightforward migration to
AbstractDeclaredStatement.WithRawStringArgument subclasses, which
ends up reducing typical footprint from 32 bytes to 16/24 bytes.
On the effective front, this does not really do anything, as these
are handled on per-type basis.
JIRA: YANGTOOLS-1065
Change-Id: I421f72a765fd6a28f984d463d8b9991dd7c1d231
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>