gcc-9 gives the following shadow warning:
```
In file included from /Users/libraries/ioss/src/fmt/ostream.h:12,
                 from /Users/libraries/ioss/src/Ioss_DatabaseIO.C:59:
/Users/libraries/ioss/src/fmt/format.h: In function 'void fmt::v6::internal::parse_format_string(fmt::v6::basic_string_view<Char>, Handler&&)':
/Users/libraries/ioss/src/fmt/format.h:2442:10: warning: declaration of 'struct fmt::v6::internal::parse_format_string(fmt::v6::basic_string_view<Char>, Handler&&)::writer' shadows a global declaration [-Wshadow]
 2442 |   struct writer {
      |          ^~~~~~
/Users/libraries/ioss/src/fmt/format.h:1703:7: note: shadowed declaration is here
 1703 | using writer = basic_writer<buffer_range<char>>;
      |       ^~~~~~
```

Since the `writer` struct is only used internally in the `parse_format_string` function, its name can be changed somewhat aribtrarily to avoid conflicts with names in an outer scope.

Note that this warning is also present in the 6.0.0 release.

to reduce the number of instantiations

to make sure we don't break the API too much when making all of this
compile-time only.