vendor: fmtlib: Upgrade fmtlib to 9.1.0

vendor: fmtlib: Upgrade fmtlib to 9.1.0

gcc warns that:

  declaration of ‘struct fmt::v8::detail::parse_format_string(fmt::v8::basic_string_view, Handler&&) [with bool IS_CONSTEXPR = false; Char = char; Handler = fmt::v8::detail::vformat_to(fmt::v8::detail::buffer&, fmt::v8::basic_string_view, fmt::v8::basic_format_args::type, char>::value, fmt::v8::appender, std::back_insert_iterator::type> > >::type, typename fmt::v8::type_identity::type> >, fmt::v8::detail::locale_ref) [with Char = char; typename fmt::v8::type_identity::type = char; typename std::conditional::type, char>::value, fmt::v8::appender, std::back_insert_iterator::type> > >::type = fmt::v8::appender]::format_handler]::writer’ shadows a global declaration [-Wshadow]
  struct writer {
  ^~~~~~
  commands/start.cpp:22:26: note: shadowed declaration is here
  static struct mi_writer *writer;
  ^~~~~~
  In file included from ../../../src/common/format.hpp:19:0,

This problem was fixed in more recent versions of fmtlib than the one we
vendorized (8.1.0): https://github.com/fmtlib/fmt/issues/1688

Upgrade the vendorized fmtlib to the latest stable release since it
fixes a number of similar warnings. However, the fix described in the
issue had to be re-applied as c06851456 (in fmtlib's tree) reintroduces
the problem.

A format_as helper function must be provided to format enums since this
version, as seen in ust-field-convert.cpp.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I5092d1c348670ec358d571f19d3598ee7662cd21