DRY logic in the module generation

[greyblake]

Context

At the moment we support 3 type families: integers, floats and String.
Recently Newtype trait was introduced, which formalizes parse -> validate -> generate flow.
The generation logic is handled individually by each single type in gen modules, by the following functions:

• gen_nutype_for_integer
• gen_nutype_for_float
• gen_nutype_for_string

Those functions are very similar and have a similar structure.
Since we're going to add new type families in the nearest future we want to find a way to avoid those duplications and DRY them.

Specs

• Find a way to DRY the duplications (consider higher order functions, traits with template methods, or something else

Acceptance Criteria

• The tests remain untouched and pass
• The code is restructured in the way that adding a new type family will require writing less code for the generation logic.

[greyblake]

I am working on it.

[greyblake]

Done in #73