Go: style changes; fixed vet/staticcheck issues; better pathing

[zacharysyoung]

This PR:

• fixes some basic issues as reported by go vet/staticcheck
• adds system-aware pathing (I believe: not on Windows, cannot vet)
• adds constants for the null value, value separator, and row separator
• adds errors that can be reused, and checked against
• made some variable naming/style changes
• changed byteClassLookup and stateTransitionLookup from slices to fixed-sized arrays; not necessary, but signals that the vars won't be changing size