Various Improvements

[empijei]

I added a few tests and refactored code to follow styles specified in go codereview comments.

In details:

• Left-aligned "happy path"
• Changed nested code in else blocks with early returns
• Changed deeply nested if to a switch
• Made test failure output expected and gotten value
• Added a test case for LRU eviction
• Added a test case to check if previously inserted values persist