yang-parser-impl supports ASTSchemaSource

yang-parser-impl supports ASTSchemaSource

yang-parser-rfc7950 supports bridging parsing from ASTSchemaSource,
which is an intermediate product of parsing from YangTextSchemaSource.

Make sure we propagate this capability from YangParserImpl, so that
users can efficiently integrate with it. While we're in the area
teach YangStatementStreamSource how to instantiate itself from
an ASTSchemaSource and add ASTSchemaSource.toString().

Change-Id: I0b6440b6fc4e7522c70261cffcef0afafed8ba53
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>