Fix -Wsign-conversion in bigint::assign.

num_bigits is initialised to 0, is only ever incremented, and is passed to basic_memory_buffer::operator[] (size_t) and basic_memory_buffer::resize (size_t).

include/fmt/format-inl.h

@@ -578,7 +578,7 @@ class bigint {
   }
 
   void assign(uint64_t n) {
-    int num_bigits = 0;
+    size_t num_bigits = 0;
     do {
       bigits_[num_bigits++] = n & ~bigit(0);
       n >>= bigit_bits;