Code structure

[nevrome]

For the CRAN release of rrtools (#75), @benmarwick and I decided to restructure its code files to increase the general readability and maintainability. This PR contains a first draft of a new file structure.

What's new:

• hello.R was divided into several code files. One file (core_...) for each main function of the package.
• The helper functions -- many copied from the devtools package to avoid ::: dependencies -- were reorganized in thematic files (helpers_...) with a more or less unified layout.
• Some helper functions, which are not in use any more, were removed.
• Helper functions are no longer exported and their (mostly internal) roxygen documentation was removed.

[benmarwick]

Thanks for this huge effort, it looks excellent. I've cloned this PR locally, built, checked, tested, etc. and it's great.

[benmarwick]

I rarely use branches, but it looks like I could merge this without wrecking anything, so that's a relief! Thanks again for all your work organising those functions. It will be easier to maintain now!