Refactoring

[jvanstraten]

Working on refactoring the following things:

• All definitions must go into a .cc file
• Everything that has to do with OpenQL must be in the ql namespace
• Basic attempt at const-correctness
• Pass non-primitive types by const-reference
• Code style normalization, mostly to do with where whitespace goes
• Fixes for obviously broken code as encountered
• Anything clang-tidy warns about that can be easily fixed
• Reverse Yoda conditions (if (red == apple))
• Remove str.h and exception.h because they came from XPU and are apparently GPL (I have some bad news about the license of QX, haha)
• Update the code style description in CONTRIBUTING.md.

TODO for later (after CC is merged), however:

• Renaming anything
• Dead code removal
  • Such as the mostly unused builtin gates
• Functional fixes
• Significant file/directory restructuring
• Making things private

[jvanstraten]

Coding style now in contributing.md is strange place; but acceptable.

That's where I found it initially, I just amended it. It should indeed go somewhere more prominent at some point.