Fix 128-bit text output for library types; use numeric_limits in MaskToNBits

`WriteIntegerToTextStream` subscripted its digit table with the value being
printed, which does not compile when `EMBOSS_UINT128_T` is overridden with a
class type such as `absl::uint128`, as `emboss_defines.h` advertises.  Cast
the digit (always below the numeric base) to `int` first.

`MaskToNBits` rejected signed types with `std::is_signed<T>`, which is false
for `__int128` under a strict `-std=c++NN`, so the check was silently skipped
there.  Use `std::numeric_limits<T>::is_signed` instead, which the runtime
already relies on for 128-bit types.

runtime/cpp/emboss_bit_util.h

diff --git a/runtime/cpp/emboss_bit_util.h b/runtime/cpp/emboss_bit_util.h
index f9b031c..d8b7a92 100644
--- a/runtime/cpp/emboss_bit_util.h
+++ b/runtime/cpp/emboss_bit_util.h
@@ -18,6 +18,7 @@
 #define EMBOSS_RUNTIME_CPP_EMBOSS_BIT_UTIL_H_
 
 #include <cstdint>
+#include <limits>
 #include <type_traits>
 
 #include "runtime/cpp/emboss_defines.h"
@@ -58,16 +59,19 @@
 
 #if EMBOSS_HAS_INT128
 inline constexpr EMBOSS_UINT128_T ByteSwap(EMBOSS_UINT128_T x) {
-  return (static_cast<EMBOSS_UINT128_T>(ByteSwap(static_cast<::std::uint64_t>(x)))
+  return (static_cast<EMBOSS_UINT128_T>(
+              ByteSwap(static_cast</**/ ::std::uint64_t>(x)))
           << 64) |
-         ByteSwap(static_cast<::std::uint64_t>(x >> 64));
+         ByteSwap(static_cast</**/ ::std::uint64_t>(x >> 64));
 }
 #endif  // EMBOSS_HAS_INT128
 
 // Masks the given value to the given number of bits.
 template <typename T>
 inline constexpr T MaskToNBits(T value, unsigned bits) {
-  static_assert(!::std::is_signed<T>::value,
+  // numeric_limits, not is_signed: the latter is false for __int128 under a
+  // strict -std=c++NN, which would silently disable this check.
+  static_assert(!::std::numeric_limits<T>::is_signed,
                 "MaskToNBits only works on unsigned values.");
   return bits < sizeof value * 8 ? value & ((static_cast<T>(1) << bits) - 1)
                                  : value;

runtime/cpp/emboss_text_util.h

diff --git a/runtime/cpp/emboss_text_util.h b/runtime/cpp/emboss_text_util.h
index 061ea31..e8dea00 100644
--- a/runtime/cpp/emboss_text_util.h
+++ b/runtime/cpp/emboss_text_util.h
@@ -307,7 +307,7 @@
         digit = 0;
         ++value;
       }
-      buffer_char(digits[digit]);
+      buffer_char(digits[static_cast<int>(digit)]);
       ++digit_count;
     } else {
       value = -value;
@@ -317,7 +317,7 @@
     if (digit_count && digit_count % grouping == 0 && digit_grouping) {
       buffer_char('_');
     }
-    buffer_char(digits[value % base]);
+    buffer_char(digits[static_cast<int>(value % base)]);
     value /= base;
     ++digit_count;
   }