Docs, doc-tests, unit-tests

[AdrienChampion]

I'm getting started on this code base and find it difficult to understand as there is no documentation or tests.

After some discussion with @lefessant I started documenting/doc-testing the code. Here is a taste of it, we would like to know if you are interested in this kind of contribution.

Note that

• most of the comments are taken from/inspired by the TON-VM description (PDF), some modifications might be needed to fit how it's actually implemented;
• the files I changed ran through rustfmt, as is routine;
• I also included remarks on some choices in the implementation: they might be best discussed elsewhere.