switch to fmtlib/fmt from boost::format

[lambdafu]

This is what everybody else seems to be using instead of boost::format, which is slow and also had some surprises up its sleeve: https://github.com/fmtlib/fmt

Replacing the occurences of boost::format with this should be straight forward. It is not a header-only library, but just one header and one source file. https://github.com/gabime/spdlog comes with a header-only version included, in case we want to use that.

[lambdafu]

We are using spdlog now, so we can us libfmt.