Move state tracking into the puller/scanner objects. This is a first
step towards resolving #1391.
Rename Puller and Scanner to roFolder and rwFolder as they have more
duties than just pulling and scanning, and don't need to be exported.
Integers are for numbers, enabling arithmetic like subtractions and for
loops without getting shot in the foot. Unsigneds are for bitfields.
- "int" for numbers that will always be laughably smaller than four
billion, and where we don't care about the serialization format.
- "int32" for numbers that will always be laughably smaller than four
billion, and will be serialized to four bytes.
- "int64" for numbers that may approach four billion or will be
serialized to eight bytes.
- "uint32" and "uint64" for bitfields, depending on required number of
bits and serialization format. Likewise "uint8" and "uint16", although
rare in this project since they don't exist in XDR.
- "int8", "int16" and plain "uint" are almost never useful.
