Add two at() functions that return mutable pointer to child nodes, based
on an index for lists and a key for dictionaries.
The API differs from const children accessors that use operator[] and
return a reference in order to allow better error handling: while the
const accessors return a reference to a global instance of an empty
ValueNode when the requested child doesn't exist, a mutable accessor
can't do the same without allowing modifying the empty global instance.
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Isaac Scott <isaac.scott@ideasonboard.com>
eafc366012