Restructure repository / packages

[liamsi]

Currently, every file lives in the root directory. We need to refactor the code such that different logical parts live in different packages. This will also help clarifying the exposed API.

[erikgrinaker]

I don't think we should do this. We might consider it for internal code (which should go in the special internal package), but I find it ergonomic and idiomatic to keep libraries like these in a single package - see e.g. BadgerDB, BoltDB, and Google B-tree as examples of Go data stores which also do this.

[tac0turtle]

closing this as IAVL is partially in maintenance, no need to change things up