[ruby/bigdecimal] Implement special conversions for 64-bit integers

This change improves the conversion speed from small integers.

```
Comparison:
                           big_n9
              master:   4003688.9 i/s
    bigdecimal 3.0.0:   1270551.0 i/s - 3.15x  slower

                          big_n19
              master:   5410096.4 i/s
    bigdecimal 3.0.0:   1000250.3 i/s - 5.41x  slower
```

https://github.com/ruby/bigdecimal/commit/3429bd7e6f

1 files changed, 54 insertions, 0 deletions

diff --git a/ext/bigdecimal/static_assert.h b/ext/bigdecimal/static_assert.h
new file mode 100644
index 0000000000..9295729bf6
--- /dev/null
+++ b/ext/bigdecimal/static_assert.h
@@ -0,0 +1,54 @@
+#ifndef BIGDECIMAL_STATIC_ASSERT_H
+#define BIGDECIMAL_STATIC_ASSERT_H
+
+#include "feature.h"
+
+#ifdef HAVE_RUBY_INTERNAL_STATIC_ASSERT_H
+# include <ruby/internal/static_assert.h>
+#endif
+
+#ifdef RBIMPL_STATIC_ASSERT
+# define STATIC_ASSERT RBIMPL_STATIC_ASSERT
+#endif
+
+#ifndef STATIC_ASSERT
+# /* The following section is copied from CRuby's static_assert.h */
+
+# if defined(__cplusplus) && defined(__cpp_static_assert)
+#  /* https://isocpp.org/std/standing-documents/sd-6-sg10-feature-test-recommendations */
+#  define BIGDECIMAL_STATIC_ASSERT0 static_assert
+
+# elif defined(__cplusplus) && defined(_MSC_VER) && _MSC_VER >= 1600
+#  define BIGDECIMAL_STATIC_ASSERT0 static_assert
+
+# elif defined(__INTEL_CXX11_MODE__)
+#  define BIGDECIMAL_STATIC_ASSERT0 static_assert
+
+# elif defined(__cplusplus) && __cplusplus >= 201103L
+#  define BIGDECIMAL_STATIC_ASSERT0 static_assert
+
+# elif defined(__cplusplus) && __has_extension(cxx_static_assert)
+#  define BIGDECIMAL_STATIC_ASSERT0 __extension__ static_assert
+
+# elif defined(__STDC_VERSION__) && __has_extension(c_static_assert)
+#  define BIGDECIMAL_STATIC_ASSERT0 __extension__ _Static_assert
+
+# elif defined(__STDC_VERSION__) && defined(__GNUC__) && (__GNUC__ > 4 || (__GNUC__ == 4 && __GNUC_MINOR__ >= 6))
+#  define BIGDECIMAL_STATIC_ASSERT0 __extension__ _Static_assert
+#endif
+
+# if defined(__DOXYGEN__)
+#  define STATIC_ASSERT static_assert
+
+# elif defined(BIGDECIMAL_STATIC_ASSERT0)
+#  define STATIC_ASSERT(name, expr) \
+    BIGDECIMAL_STATIC_ASSERT0(expr, #name ": " #expr)
+
+# else
+#  define STATIC_ASSERT(name, expr) \
+    typedef int static_assert_ ## name ## _check[1 - 2 * !(expr)]
+# endif
+#endif /* STATIC_ASSERT */
+
+
+#endif /* BIGDECIMAL_STATIC_ASSERT_H */