Rm db.Info, rm go-iden3-core dependency, update version of go-iden3-crypto
Add pebble Batch implementation for db.Tx, and update db.Tx Put & Add methods to return error.
Add own db (storage) package with db.Storage interface, with Memory & Leveldb implementations, and with tests. Adapted with several changes from https://github.com/iden3/go-iden3-core/tree/v0.0.8/db