Restructure library

[mialbu]

For some types, there's a distinct file (e.g., Order struct in order.go) while other types are collected in the file opensea-model.go (e.g., Asset, or AssetContract). The structure should be more consistent throughout the library.

Suggestion: Create a file stat.go for Stat, asset.go for Asset and AssetContract, and collection.go for Collection.

Further, create a util file that holds different utility methods that are currently also held by opensea-model.go.