Add C++ "to_string()" function to declarations.

Cython/Includes/libcpp/string.pxd

@@ -173,3 +173,14 @@ cdef extern from "<string>" namespace "std" nogil:
 
         bint operator>= (const string&)
         bint operator>= (const char*)
+
+
+    string to_string(int val)
+    string to_string(long val)
+    string to_string (long long val)
+    string to_string (unsigned val)
+    string to_string (unsigned long val)
+    string to_string (unsigned long long val)
+    string to_string (float val)
+    string to_string (double val)
+    string to_string (long double val)

tests/run/cpp_stl_string.pyx

@@ -3,7 +3,7 @@
 
 cimport cython
 
-from libcpp.string cimport string
+from libcpp.string cimport string, to_string
 
 b_asdf = b'asdf'
 b_asdg = b'asdg'
@@ -346,6 +346,18 @@ def test_iteration(string s):
     return [c for c in s]
 
 
+def test_to_string(x):
+    """
+    >>> print(test_to_string(5))
+    si=5 sl=5
+    >>> print(test_to_string(-5))
+    si=-5 sl=-5
+    """
+    si = to_string(<int>x).decode('ascii')
+    sl = to_string(<long>x).decode('ascii')
+    return f"si={si} sl={sl}"
+
+
 _WARNINGS = """
 21:31: Cannot pass Python object as C++ data structure reference (string &), will pass by copy.
 """