Feature tower

[riraccuia]

• Add license file (MIT)
• Implement Tower and Level structures to match the inner workings of a skip list
• Consolidate the SkipList structure, constructor and methods implementation, atomically proofing for concurrent usage
• Implement the Map structure as a drop-in replacement for golang's sync.Map backed by skip list, it replicates the same exact signature with the addition of a Len() method
• Allow overrides for the random height function
• Start documenting, with added guidance on tuning the maxlevel value