[ag] generated elements in a separated file ?

[GuillaumeDua]

• Goal: avoid misleading changes in generated code
  • default: in-source
  • opt-in: in a dedicated file
  • or ... keep everything at the end of the csl/ag.hpp ?
• move (most of) the generated code to a dedicated file, to keep csl/ag.hpp clean
• Yet, let a support for only a few elements (8 ?) in csl/ag.hpp,
  to preserve stand-alone/single-header usage in most cases.

/* etc... */
#if defined __has_include
#  if __has_include (<csl/ag/injections.hpp>)
#    include <csl/ag/injections.hpp>
#  endif
#endif
/* etc... */