Re-structure modules for more reasonable layers' and components' separation

[wprzytula]

As we're heading towards 1.0, we should clean the module structure up as well.

There is especially one offender: the transport module, which contains essentially everything; the very name transport says nothing about its content.

In the following Doc, I propose a new structure of modules, along with rationale for that and a detailed structure of exports.
https://docs.google.com/document/d/1OalpdKhwJK0CrPjWg2-CTzi7vYtIO3AZG-IbeyYOC4g/edit?usp=sharing

The work described should be done together with considering the prelude module intended contents (#637).