Add fmtlib dependency

[feltech]

What

Add fmtlib (private) dependency.

Why

fmt allows for easy, safe string substitution in C++, adding advanced formatting functionality (e.g. rounding), and reducing bloat, e.g.

    std::string message = "Parameter lists must be of the same length: ";
    message += std::to_string(entityReferences.size());
    message += " entity references vs. ";
    message += std::to_string(traitsHints.size());
    message += " traits hints.";
    throw std::out_of_range{message};

Formatting user-defined types can be added in a variety of ways with fmt. Perhaps most portable is operator<< support.

However, fmt already supports several types, including container types, out of the box, so hooks for custom types might not be necessary at this point.

ACs

• fmtlib installed via Conan.
• fmtlib as a private, header-only dependency in CMake
• Brief check of the out-of-the box support for printing our types (e.g. TraitSet)
• Update a single string somewhere to prove it works

Out of scope

• Updating existing string formatting
• Implementing complex user-defined type formatting hooks.

[feltech]

Added #1075 make broader use of fmtlib

[feltech]

Posted upstream re. unexpected forcibly exported symbol: fmtlib/fmt#3626