Use type annotation to make clear a constant

I’m thinking of using a type annotation like in this case to make clear
what the constant 1 is. In this case it should be clear that I’m
passing a tag number and a length to writeCSTag
Implement and test writeNullCS
Implement and test readUnsignedCS
Add instance CSEncodable Null
Refactor tag readers out of Tag.hs

Tag.hs is getting too big and messy. This first step pulls out some
of the core stuff into BACnet.Tag.Core.hs and all the readers
into BACnet.Tag.Reader.hs