Refactor ExplicitStatement
Rather than having a superclass, just create StatementDeclarations as a
host for the two factory methods. Implementations are completely
internal and do not leak outside of this class.
We also optimize memory footprint by using two versions of each class --
one storing file/line as ints, the other one as shorts. This ends up
saving a few bytes of memory.
Eliminate the InFile specialization, as it is not used anywhere really.
Also renale atPosition() to inText() to more closely align naming.
JIRA: YANGTOOLS-1150
Change-Id: Ib5a20e45f645f00fac8d1d27c48dd4dcab16fb17
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>