Changing type of data_ to size_t to avoid compilation warnings (#4200)

Changing type data_ to size_t because
1. If lib is cross-compiled for win32 using MXE environment it cause
   compilation warning -Wconversion on line 730 as sizeof(unsigned long)
   = 4 and sizeof(size_t) = 8
2. When lib is compiled on Unix like compiler generate warning
   -Wuseless-cast if static_cast is used to fix issue in 1

include/fmt/base.h

@@ -721,7 +721,7 @@ class basic_specs {
     max_fill_size = 4
   };
 
-  unsigned long data_ = 1 << fill_size_shift;
+  size_t data_ = 1 << fill_size_shift;
 
   // Character (code unit) type is erased to prevent template bloat.
   char fill_data_[max_fill_size] = {' '};