Declare strict types and enforce them, fixes #679

[Zerquix18]

Here I declare strict types and also enforce them. It fixes #679 and helps with #678

Things to consider:

• Some functions take several types of params (e.g array and objects). I think we can try to reduce them to just one. I did not set a type for those params.
• Some functions return several types of params (e.g array and objects). I think we can we just one as well. I dislike the idea that sometimes it returns an object and sometimes an array, and this has broken my code sometimes. We can open a separate issue for these.

Hope this contribution is useful 😄

[abraham]

Awesome! Thank you. I'll get this merged one v1 is released.

[abraham]

Thanks!