Remove some implicit conversions (#4447)

* fix: avoid an implicit cast

The "1" used for the bitshift is treated as int, and this causes an
implicit conversion to `UInt` when performing the logical and.
Explicitly casting the number to `UInt` avoids the warning.

* fix: avoid implicit conversions for indices

Some indices in `include/fmt/base.h` are expressed as `int` types, which
causes an implicit conversion to a `size_t` when they are actually used
as index. Explicitly casting the value avoids the warning.

* fix: avoid an implicit conversion using size_t

The number of bits is used to express the size of a buffer. Using an
`int` causes an implicit conversion warning, let's use a `size_t` which
is the right type for the job.

include/fmt/format.h

@@ -1225,7 +1225,8 @@ FMT_CONSTEXPR auto do_format_base2e(int base_bits, Char* out, UInt value,
   out += size;
   do {
     const char* digits = upper ? "0123456789ABCDEF" : "0123456789abcdef";
-    unsigned digit = static_cast<unsigned>(value & ((1 << base_bits) - 1));
+    unsigned digit = static_cast<unsigned>(
+        value & ((static_cast<UInt>(1) << base_bits) - 1));
     *--out = static_cast<Char>(base_bits < 4 ? static_cast<char>('0' + digit)
                                              : digits[digit]);
   } while ((value >>= base_bits) != 0);
@@ -2017,7 +2018,7 @@ FMT_CONSTEXPR FMT_INLINE auto write_int(OutputIt out, write_int_arg<T> arg,
                                         const format_specs& specs) -> OutputIt {
   static_assert(std::is_same<T, uint32_or_64_or_128_t<T>>::value, "");
 
-  constexpr int buffer_size = num_bits<T>();
+  constexpr size_t buffer_size = num_bits<T>();
   char buffer[buffer_size];
   if (is_constant_evaluated()) fill_n(buffer, buffer_size, '\0');
   const char* begin = nullptr;