Implement GIL-free string formatting

src/stdlib/format.pxd

+from stdlib.string cimport Str
+
+
+
+cdef extern from * nogil:
+    """
+    #include <fmt/format.h>
+    #include <string_view>
+
+    template <typename T, typename S>
+    struct Cy_Str_Adapter {
+        using type = T;
+        constexpr static auto adapt(T t) { return t; }
+    };
+
+    template <typename T>
+    struct Cy_Str_Adapter<T*, T> {
+        using type = std::string_view;
+        static std::string_view adapt(T* t) {
+            return t->operator std::string_view();
+        }
+    };
+
+    template <typename S, typename... T>
+    S * format(fmt::format_string<typename Cy_Str_Adapter<T, S>::type...> fmt, T... args) {
+        S *s = new S();
+        s->_str = fmt::format(fmt, Cy_Str_Adapter<T, S>::adapt(args)...);
+        return s;
+    }
+    """
+
+    Str format "format<Cy_Str>" (const char *, ...) except +
+

src/stdlib/string.pxd

@@ -22,7 +22,7 @@ cdef extern from "<cctype>" namespace "std" nogil:
 
 
 
-cdef cypclass Str:
+cdef cypclass Str "Cy_Str":
     string _str
 
     __init__(self, const char *s):
@@ -137,6 +137,9 @@ cdef cypclass Str:
     int __int__(self) except +:
         return stoi(self._str)
 
+    string_view __string_view__(self):
+        return string_view(self._str.data(), self._str.size())
+
     @staticmethod
     const char * to_c_str(Str s):
         if s is NULL: