[libc] Fix -Wimplicit-int-conversion warnings

libc/src/__support/StringUtil/message_mapper.h

template <size_t ARR_SIZE, size_t TOTAL_STR_LEN> class MessageMapper {
   char string_array[TOTAL_STR_LEN] = {'\0'};
 
 public:
   constexpr MessageMapper(const MsgMapping raw_array[], size_t raw_array_len) {
     cpp::string_view string_mappings[ARR_SIZE] = {""};
     for (size_t i = 0; i < raw_array_len; ++i)
       string_mappings[raw_array[i].num] = raw_array[i].msg;
 
-    size_t string_array_index = 0;
+    int string_array_index = 0;
     for (size_t cur_num = 0; cur_num < ARR_SIZE; ++cur_num) {
       if (string_mappings[cur_num].size() != 0) {
         msg_offsets[cur_num] = string_array_index;
         // No need to replace with proper strcpy, this is evaluated at compile
         // time.
         for (size_t i = 0; i < string_mappings[cur_num].size() + 1;
              ++i, ++string_array_index) {
           string_array[string_array_index] = string_mappings[cur_num][i];

libc/src/__support/integer_to_string.h

static cpp::string_view convert_uintmax(uintmax_t uval,
 
     size_t buffptr = buffer.size();
     if (uval == 0) {
       buffer[buffptr - 1] = '0';
       --buffptr;
     } else {
       for (; uval > 0; --buffptr, uval /= conv_base) {
         uintmax_t digit = (uval % conv_base);
-        buffer[buffptr - 1] = digit < 10 ? digit + '0' : digit + a - 10;
+        buffer[buffptr - 1] = static_cast<char>(digit < 10 ? digit + '0' : digit + a - 10);
       }
     }
     len = buffer.size() - buffptr;
 
     return cpp::string_view(buffer.data() + buffer.size() - len, len);
   }
 
   static cpp::string_view convert_intmax(intmax_t val, cpp::span<char> &buffer,