Move module tests to the tests/ folder

[newpavlov]

This improves visibility of the tests and makes the modules less cluttered.

[dhardy]

For lib.rs I agree that it's too cluttered. I considered moving the traits to sub-modules but at the time this would have conflicted with other changes; moving only the tests out is also fine.

For the other sub-modules, I don't see a strong motivation (especially utils which had few tests).

[newpavlov]

For the other sub-modules, I don't see a strong motivation (especially utils which had few tests).

I plan to add several new tests for the utils functions to improve coverage. In general, I prefer to keep public API tests in the tests/ folder and use unit tests (i.e. mod tests) only for private APIs.