Switch to C++14, drop make_unique.hh
Improve Introduction in the documentation
Use brief output for string interpolation

- So that a "%s" doesn't end up formatting whole DIE.
Cleanups
Rehaul of formal documentation

- A bunch of things were moved from language parts of the
  documentation to new sections dedicated to dictionaries.

- Another bunch of things where we are more or less clear on what
  should be done, were moved from TODO parts to dictionaries as well,
  and marked as not done (or partially done, or sometimes as fully
  done, such as some of the location expression stuff).