Restructure project

We previously didn't really had any structure in our project apart
from creating a new folder for each package in our project root.
Now that we have accumulated some packages, we use the well-known
Golang project layout in order to clearly communicate our intent
with packages. See https://github.com/golang-standards/project-layout

.gitignore

@@ -1,5 +1,5 @@
 # Project binary
-poseidon
+/poseidon
 
 # Configuration file
 configuration.yaml