Support migration to datomic, support for saving datoms in nippy, refactoring of core

[jonasseglare]

This PR makes the following contributions to Wanderung:

• A new function transact-datoms-to-datomic for putting data into datomic
• Support for migrating datoms to and from nippy-encoded files: this is probably a suitable backup format.
• Refactoring of wanderung.core and the commandline interface.

An important change that I made to the interface of the program is that instead of passing a -d or --direction option specifying the type of source and target databases on the commandline, the type of database is instead stored under the key :wanderung/type in the database configuration. I am happy to discuss whether this is a good or bad idea. So for instance, the config of a datomic database now looks like this:

{:wanderung/type :datomic
 :name "your-database"
 :server-type :ion
 :region      "eu-west-1"
 :system      "your-system"
 :endpoint    "http://entry.your-system.eu-west-1.datomic.net:8182/"
 :proxy-port  8182}

and, for example, the configuration for a file that stores nippy-encoded datoms looks like this:

{:wanderung/type :nippy
 :filename "backups/mybackup.nippy"}

The main reason for this change is that it leaves less room for mistakes since it is no longer possible to specify a direction with the -d flag that is inconsistent with the configurations.

We wrote this code at Jobtech to be able to create and restore backups of our Taxonomy database.

[jonasseglare]

Including this file in the project makes it possible to refer to the project from deps.edn, e.g. io.lambdaforge/wanderung {:git/url "https://github.com/jonasseglare/wanderung" :sha "fe35e0282f787d7f519acc9201ee99eb3eb881ec"}}.

[kordano]

@jonasseglare looks good. Thanks for your contribution!