Add string formatting

format.pxd

+# distutils: libraries = fmt
+
+from .string cimport Str
+
+# Description:
+# This defines a format() function that accepts Str arguments for formatting
+# and returns a Str. The format string itself must still be a string literal.
+#
+# Explanation:
+# Cy_Str is a template parameter in this C++ code only because this code is
+# inserted before the definition of Cy_Str (C++ name of cypclass Str) in the
+# generated C++ output. The actual Cy_Str type is injected at each call site
+# of the format function. This is achieved by explicitly setting the C++ name
+# of the format function to `format<Cy_Str>`.
+cdef extern from * nogil:
+    """
+    #include <fmt/format.h>
+    #include <string_view>
+
+    template <typename T, typename Cy_Str>
+    struct Cy_Str_Adapter {
+        constexpr static auto adapt(T t) { return t; }
+    };
+
+    template <typename Cy_Str>
+    struct Cy_Str_Adapter<Cy_Str*, Cy_Str> {
+        static std::string_view adapt(Cy_Str* s) {
+            return s->operator std::string_view();
+        }
+    };
+
+    template <typename Cy_Str, typename Fmt, typename... T>
+    Cy_Str * format(const Fmt & fmt, T... args) {
+        Cy_Str *s = new Cy_Str();
+        s->_str = fmt::format(fmt, Cy_Str_Adapter<T, Cy_Str>::adapt(args)...);
+        return s;
+    }
+    """
+
+    Str format "format<Cy_Str>" (const char *, ...) except +
+