Split up Parquet Array Reader Module

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

The parquet/arrow/array_reader.rs is currently sitting pretty at 2984 lines of code. Not only does this make the file difficult to navigate, but it also makes it harder to tell what components have unit test coverage - as there is just a large wall of unit tests at the bottom of the file.

Describe the solution you'd like

New functionality, such as ByteArrayReader, has already been split into child modules. We should do the same for StructArrayReader, MapArrayReader, NullArrayReader, etc...

Describe alternatives you've considered

The module could be left unchanged.

Additional context

#1246 discovered a lack of any testing of NullArrayReader