Support reading large values dynamically

[tgregg]

ion-c's reader includes configurable limits on value sizes. See _ion_reader_options.symbol_threshold and _ion_reader_options.user_value_threshold. The configured values result in fixed-size internal buffers that are incapable of handling Ion values that exceed the configured limits if such values occur while reading a stream. This is unsuitable in for use cases that are not aware of, or do not control, the size of values that may be encountered.