migrate src/utils into its own library

[magiruuvelvet]

There are a lot of useful and reusable functions in there. src/utils doesn't depend on the remaining codebase (except some cmake-specific shenanigans*), so it should be generally easy to split it into its own dedicated library.

* operating system detection macros (using a build system for this is more reliable and portable than depending on weird built-in compiler-specific macros)