Code documentation: Utils.

This is a header-only library, containing various utility functions, such as:

bounded_arithmetic.h: multiplication and addition, capped at a max value
concat.h: vararg function to concatenate many values to a string
deprecate_header.h: macro deprecating a header
enumeration_utils.h: to_underlying (backported from C++23), to_enumeration
error.h: definition of Error - base class for (C++) error handling
is_specialization.h: utility to determine if a template is a specialization of another.
nodiscard.h: replacement for [[nodiscard]] that works around gcc bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84476
return_status.h: enum { ok, error }