Logging overhaul, fmt::format, and related

[jagerman]

This replaces custom logging system with spdlog-based oxen logging. This PR mainly replaces the backend logging with the spdlog-based system, but doesn't (yet) convert all the existing LogWarn, etc. to use the new format-based logging.

New logging statements will look like:

llarp::log::warning(cat, "blah: {}", val);

where cat should be set up in each .cpp or cluster of .cpp files, as described in the oxen-logging README

As part of spdlog we get fmt, which gives us nice format strings, where are applied generously in this PR.

Making types printable now requires two steps:

• add a ToString() method

• add this specialization:

  template <> constexpr inline bool llarp::IsToStringFormattable = true;

This will then allow the type to be printed as a "{}" value in a fmt::format string. This is applied to all our printable types here, and all of the operator<< are removed.

This commit also:

• replaces various uses of operator<< to ToString()
• replaces various uses of std::stringstream with either fmt::format or plain std::string
• Rename some to_string and toString() methods to ToString() for consistency (and to work with fmt)
• Replace stringify(...) and make_exception usage with fmt::format (and remove stringify/make_exception from util/str.hpp).

[jagerman]

Still todo:

• [x] macos support
• [x] get rid of the .print(blah, -1, -1) print layer