Split HTF into several packages about htf

[lslah]

We could start adding separate library entries to HTF.cabal for all of the suggested sub-packages. If at some point a library only depends on other libraries and its own modules, it should be easy to extract the whole thing into a separate package.

[skogsbaer]

Seems like a good start. I would suggest that we do the restructuring without adding any new functionality or changing functionality. For example, we just leave the preprocessor "as is" and add the config file later. With this approach, we can run the tests at the end and make sure that everything works as before.

I think the dependencies should be as follows:

• HTF-core: only external dependencies
• HTF-preprocessor: only external dependencies (the generated code has more dependencies)
• HTF-UnitTests, HTF-QuickCheck, HTF-ShellTests: on HTF-core
• HTF: HTF-core, HTF-preprocessor, HTF-UnitTests, HTF-QuickCheck, HTF-ShellTests:

[skogsbaer]

Closing this because I won't fix it. I'm happy to accept PRs, though.