Macro std_tools::tests_impls_optional

macro_rules! tests_impls_optional {
    () => { ... };
    (
      $( #[ $Meta : meta ] )*
      $Vis : vis
      fn $Name : ident
      $( $Rest : tt )*
    ) => { ... };
    (
      @DefineFn
      @Meta{ $( #[ $Meta : meta ] )* }
      @Vis{ $Vis : vis }
      @Name{ $Name : ident }
      @Rest
        $Item : item
        $( $Rest : tt )*
    ) => { ... };
}

Define implementation putting each function under a macro and adding attribute #[ test ].

Use index! to generate code for each elment. Unlike elements of [test_impls!], elements of [test_impls_optional] are optional to be used in index!.