Add StmtContext.findSubstatementArgument()

A lot of our logic is implemented in terms of searching effective statements, mostly through StmtContextUtils methods. These go back way to Java 7 and those early days. We currently have Java 11 and possibility of default methods – and since these two reside in the same package the natural thing is to migrate over.

Optional<Boolean> findFirstSubstatementArgument(ConfigurationEffectiveStatement.class);

is what YANGTOOLS-783 is all about, I think.

This is a question InferredStatementContext can answer without materializing its substatements – at least for non-SchemaTreeEffectiveStatements we can just forward to prototype. For SchemaTreeEffectiveStatements we do a our->prototype namespace translation (the prototype knows its namespace) and again defer to prototype.

Let us also define a:

StmtContext.hasSubstatement(Class<? extends EffectiveStatement>)

Methods which need to use this method are:

StmtContextUtils.containsPresenceSubStmt()
StmtContextUtil.hasAncestorOfTypeWithChildOfType()
AbstractAugmentStatementSupport.hasWhenSubstatement()