Fix #117: compilation warning with gcc 6.3.0

Fix the following warning:
```
/arrow/cpp/src/arrow/vendored/fast_float/digit_comparison.h:62:50: error: right shift count >= width of type [-Werror=shift-count-overflow]
       am.power2 = int32_t((bits & exponent_mask) >> binary_format<T>::mantissa_explicit_bits());
                           ~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
```

Author: pitrou

include/fast_float/digit_comparison.h

@@ -44,40 +44,24 @@ fastfloat_really_inline int32_t scientific_exponent(parsed_number_string& num) n
 // this converts a native floating-point number to an extended-precision float.
 template <typename T>
 fastfloat_really_inline adjusted_mantissa to_extended(T value) noexcept {
+  using equiv_uint = typename binary_format<T>::equiv_uint;
+  constexpr equiv_uint exponent_mask = binary_format<T>::exponent_mask();
+  constexpr equiv_uint mantissa_mask = binary_format<T>::mantissa_mask();
+  constexpr equiv_uint hidden_bit_mask = binary_format<T>::hidden_bit_mask();
+
   adjusted_mantissa am;
   int32_t bias = binary_format<T>::mantissa_explicit_bits() - binary_format<T>::minimum_exponent();
-  if (std::is_same<T, float>::value) {
-    constexpr uint32_t exponent_mask = 0x7F800000;
-    constexpr uint32_t mantissa_mask = 0x007FFFFF;
-    constexpr uint64_t hidden_bit_mask = 0x00800000;
-    uint32_t bits;
-    ::memcpy(&bits, &value, sizeof(T));
-    if ((bits & exponent_mask) == 0) {
-      // denormal
-      am.power2 = 1 - bias;
-      am.mantissa = bits & mantissa_mask;
-    } else {
-      // normal
-      am.power2 = int32_t((bits & exponent_mask) >> binary_format<T>::mantissa_explicit_bits());
-      am.power2 -= bias;
-      am.mantissa = (bits & mantissa_mask) | hidden_bit_mask;
-    }
+  equiv_uint bits;
+  ::memcpy(&bits, &value, sizeof(T));
+  if ((bits & exponent_mask) == 0) {
+    // denormal
+    am.power2 = 1 - bias;
+    am.mantissa = bits & mantissa_mask;
   } else {
-    constexpr uint64_t exponent_mask = 0x7FF0000000000000;
-    constexpr uint64_t mantissa_mask = 0x000FFFFFFFFFFFFF;
-    constexpr uint64_t hidden_bit_mask = 0x0010000000000000;
-    uint64_t bits;
-    ::memcpy(&bits, &value, sizeof(T));
-    if ((bits & exponent_mask) == 0) {
-      // denormal
-      am.power2 = 1 - bias;
-      am.mantissa = bits & mantissa_mask;
-    } else {
-      // normal
-      am.power2 = int32_t((bits & exponent_mask) >> binary_format<T>::mantissa_explicit_bits());
-      am.power2 -= bias;
-      am.mantissa = (bits & mantissa_mask) | hidden_bit_mask;
-    }
+    // normal
+    am.power2 = int32_t((bits & exponent_mask) >> binary_format<T>::mantissa_explicit_bits());
+    am.power2 -= bias;
+    am.mantissa = (bits & mantissa_mask) | hidden_bit_mask;
   }
 
   return am;

include/fast_float/float_common.h

@@ -5,6 +5,7 @@
 #include <cstdint>
 #include <cassert>
 #include <cstring>
+#include <type_traits>
 
 #if (defined(__x86_64) || defined(__x86_64__) || defined(_M_X64)   \
        || defined(__amd64) || defined(__aarch64__) || defined(_M_ARM64) \
@@ -219,6 +220,8 @@ constexpr static float powers_of_ten_float[] = {1e0, 1e1, 1e2, 1e3, 1e4, 1e5,
                                                 1e6, 1e7, 1e8, 1e9, 1e10};
 
 template <typename T> struct binary_format {
+  using equiv_uint = typename std::conditional<sizeof(T) == 4, uint32_t, uint64_t>::type;
+
   static inline constexpr int mantissa_explicit_bits();
   static inline constexpr int minimum_exponent();
   static inline constexpr int infinite_power();
@@ -232,6 +235,9 @@ template <typename T> struct binary_format {
   static inline constexpr int smallest_power_of_ten();
   static inline constexpr T exact_power_of_ten(int64_t power);
   static inline constexpr size_t max_digits();
+  static inline constexpr equiv_uint exponent_mask();
+  static inline constexpr equiv_uint mantissa_mask();
+  static inline constexpr equiv_uint hidden_bit_mask();
 };
 
 template <> inline constexpr int binary_format<double>::mantissa_explicit_bits() {
@@ -339,6 +345,33 @@ template <> inline constexpr size_t binary_format<float>::max_digits() {
   return 114;
 }
 
+template <> inline constexpr binary_format<float>::equiv_uint
+    binary_format<float>::exponent_mask() {
+  return 0x7F800000;
+}
+template <> inline constexpr binary_format<double>::equiv_uint
+    binary_format<double>::exponent_mask() {
+  return 0x7FF0000000000000;
+}
+
+template <> inline constexpr binary_format<float>::equiv_uint
+    binary_format<float>::mantissa_mask() {
+  return 0x007FFFFF;
+}
+template <> inline constexpr binary_format<double>::equiv_uint
+    binary_format<double>::mantissa_mask() {
+  return 0x000FFFFFFFFFFFFF;
+}
+
+template <> inline constexpr binary_format<float>::equiv_uint
+    binary_format<float>::hidden_bit_mask() {
+  return 0x00800000;
+}
+template <> inline constexpr binary_format<double>::equiv_uint
+    binary_format<double>::hidden_bit_mask() {
+  return 0x0010000000000000;
+}
+
 template<typename T>
 fastfloat_really_inline void to_float(bool negative, adjusted_mantissa am, T &value) {
   uint64_t word = am.mantissa;