Refactor format_specs for #1109 and #940

Refactor `format_specs` and related APIs to support variable-width fill
(#1109), improve naming consistency, remove legacy setters (#940), and
optimize layout.

include/fmt/chrono.h

@@ -715,7 +715,7 @@ struct chrono_formatter {
 template <typename Rep, typename Period, typename Char>
 struct formatter<std::chrono::duration<Rep, Period>, Char> {
  private:
-  align_spec spec;
+  basic_format_specs<Char> spec;
   int precision;
   typedef internal::arg_ref<Char> arg_ref_type;
   arg_ref_type width_ref;
@@ -744,9 +744,9 @@ struct formatter<std::chrono::duration<Rep, Period>, Char> {
     }
 
     void on_error(const char* msg) { FMT_THROW(format_error(msg)); }
-    void on_fill(Char fill) { f.spec.fill_ = fill; }
-    void on_align(alignment align) { f.spec.align_ = align; }
-    void on_width(unsigned width) { f.spec.width_ = width; }
+    void on_fill(Char fill) { f.spec.fill[0] = fill; }
+    void on_align(align_t align) { f.spec.align = align; }
+    void on_width(unsigned width) { f.spec.width = width; }
     void on_precision(unsigned precision) { f.precision = precision; }
     void end_precision() {}
 
@@ -804,7 +804,7 @@ struct formatter<std::chrono::duration<Rep, Period>, Char> {
     using range = internal::output_range<decltype(ctx.out()), Char>;
     internal::basic_writer<range> w(range(ctx.out()));
     internal::handle_dynamic_spec<internal::width_checker>(
-        spec.width_, width_ref, ctx, format_str.begin());
+        spec.width, width_ref, ctx, format_str.begin());
     internal::handle_dynamic_spec<internal::precision_checker>(
         precision, precision_ref, ctx, format_str.begin());
     if (begin == end || *begin == '}') {

include/fmt/format-inl.h

@@ -763,7 +763,7 @@ char* sprintf_format(Double value, internal::buffer<char>& buf,
   char format[max_format_size];
   char* format_ptr = format;
   *format_ptr++ = '%';
-  if (spec.has(HASH_FLAG) || !spec.type) *format_ptr++ = '#';
+  if (spec.alt || !spec.type) *format_ptr++ = '#';
   if (spec.precision >= 0) {
     *format_ptr++ = '.';
     *format_ptr++ = '*';