Refactor code to use fewer circular imports

[JukkaL]

The implementation uses circular imports more than is necessary.

Remove any circular imports that don't increase the readability/maintainability of the code.

[JukkaL]

The module strconv is also associated with nodes.

[JukkaL]

I have removed these circular dependencies (not pushed yet):

• messages -> checker
• cgen -> build

[JukkaL]

Also had to move checker.BasicTypes to mtypes. This removed many circular dependencies (not pushed yet).

[JukkaL]

Now the changes I mentioned in the above 2 comments are available at the public repo.