Change bytestring to use type wrapper py::bytes<std::string>

py::bytes<T> will work with any T which has these methods
const char* data() and int size()

Other types can be accommodated by specializing bytes<T>
and type_caster<bytes<T>>

include/pybind11/cast.h

@@ -378,15 +378,15 @@ public:
 #endif
 };
 
-template <> class type_caster<bytestring> : public type_caster<std::string> {
+template <typename T> class type_caster<bytes<T>> : public type_caster<T> {
 public:
-    static PyObject *cast(const bytestring &src, return_value_policy /* policy */, PyObject * /* parent */) {
+    static PyObject *cast(const bytes<T> &src, return_value_policy /* policy */, PyObject * /* parent */) {
-        return PYBIND11_FROM_STRING_AND_SIZE(src.c_str(), src.size());
+        return PYBIND11_FROM_STRING_AND_SIZE(src.data(), src.size());
     }
 #if PY_MAJOR_VERSION >= 3
-    PYBIND11_TYPE_CASTER(bytestring, "bytes");
+    PYBIND11_TYPE_CASTER(bytes<T>, "bytes");
 #else
-    PYBIND11_TYPE_CASTER(bytestring, "str");
+    PYBIND11_TYPE_CASTER(bytes<T>, "str");
 #endif
 };
 

include/pybind11/pytypes.h

@@ -259,13 +259,14 @@ inline iterator handle::end() { return iterator(nullptr); }
     PYBIND11_OBJECT(Name, Parent, CheckFun) \
     Name() : Parent() { }
 
-class bytestring : public std::string {
+template <typename T>
+class bytes : public T {
 public:
-    using std::string::string;
+    using T::T;
 
-    bytestring(const std::string& src) : std::string(src) { }
+    bytes(const T& src) : T(src) { }
-    bytestring(std::string&& src) : std::string(std::move(src)) { }
+    bytes(T&& src) : T(std::move(src)) { }
-    bytestring() : std::string() { }
+    bytes() : T() { }
 };
 
 class str : public object {