Managed Expressions

[aokellermann]

Changes

• Implements managed expressions
• Closes #574

Examples

To verify release is actually working (and not just releasing at program exit):

void setManageInstance([[maybe_unused]] WolframLibraryData libData, mbool mode, mint id) {
  std::string time = std::to_string(std::time(nullptr));
  if (mode == 0) {
    sets_.emplace(id, nullptr);
    std::system((std::string("echo \"create ") + time + "\" >> /tmp/sr").c_str());
  } else {
    sets_.erase(id);
    std::system((std::string("echo \"delete ") + time + "\" >> /tmp/sr").c_str());
  }
}

$ ./test.wls eventDeduplication && cat /tmp/sr
create 1609878542
delete 1609878542
create 1609878542
delete 1609878542
create 1609878542
delete 1609878542
...
create 1609878566
delete 1609878566
create 1609878566
delete 1609878566
create 1609878566
delete 1609878566

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This change is 

[aokellermann]

@daneelsan I'm not sure why but your comments aren't showing up in reviewable for me...

Should [[maybe_unused]] be removed here?

Done.