switch back to C++11

C++17 was not gaining us significant functionality, for how much portability we
lost in using it.

This commit includes the following:

  1. Removal of std::optional. This was not gaining us any clarity.
  2. Remove structured decomposition. This is a C++17 feature.
  3. Better Node hierarchy functionality (rule-of-4.5 etc).
  4. Removal of some overly general structure in Symtab.
  5. Removal of explicit on constructors. Not so useful.
  6. Removal of std::shared_ptr. Causing lots of mess and not helping much.
  7. Removal of unhelpful cases of noexcept.