Split arrow/reader.rs module into smaller modules.

[blackmwk]

Is your feature request related to a problem or challenge?

This file is too large and more than 5k lines.

Describe the solution you'd like

We need to split it into smaller modules so that it's more maintainable.

Willingness to contribute

I can contribute to this feature independently

[mbutrovich]

Thanks for opening this, @blackmwk! I agree it's gotten rather unwieldy. Happy to contribute to this, but if you beat me to it, I can review as well.