Add StmtContext.findFirstSubstatementArgument()
Introduce utility methods to StmtContext, so that individual
implementations can provide optimized version, which does not
walk all children.
InferredStatementContext used this facility to defer to protype,
side-stepping the need to materialize substatements.
JIRA: YANGTOOLS-1157
Change-Id: I9e618d3a51f68ed03a76c718b657be38e5cedb5d
Signed-off-by: miroslav.kovac <miroslav.kovac@pantheon.tech>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>
(cherry picked from commit
c31a90ed7c04bc622c9404a4a6ef4a39b1b8648d)