First iteration
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The simplest way to implement our database for our first iteration is a wrapper of std::collection hashmap.

To do this wrapping we are going to create the concept of our database in a struct. This way, for future iterations we can change the database to a real on-disk one and the rest of the code won't even notice a difference since all methods and possible traits are going to be defined for this wrapping struct.

Then we can implement some basic methods for this struct which are going to be used by future crates: new, put and get.

Second iteration
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