add support for std::wstring following std::string (#562)

include/nanobind/stl/wstring.h

@@ -0,0 +1,39 @@
+/*
+    nanobind/stl/string.h: type caster for std::string
+
+    Copyright (c) 2022 Wenzel Jakob
+
+    All rights reserved. Use of this source code is governed by a
+    BSD-style license that can be found in the LICENSE file.
+*/
+
+#pragma once
+
+#include <nanobind/nanobind.h>
+#include <string>
+
+NAMESPACE_BEGIN(NB_NAMESPACE)
+NAMESPACE_BEGIN(detail)
+
+template <> struct type_caster<std::wstring> {
+    NB_TYPE_CASTER(std::wstring, const_name("str"))
+
+    bool from_python(handle src, uint8_t, cleanup_list *) noexcept {
+        Py_ssize_t size;
+        const wchar_t *str = PyUnicode_AsWideCharString(src.ptr(), &size);
+        if (!str) {
+            PyErr_Clear();
+            return false;
+        }
+        value = std::wstring(str, (size_t) size);
+        return true;
+    }
+
+    static handle from_cpp(const std::wstring &value, rv_policy,
+                           cleanup_list *) noexcept {
+        return PyUnicode_FromWideChar(value.c_str(), value.size());
+    }
+};
+
+NAMESPACE_END(detail)
+NAMESPACE_END(NB_NAMESPACE)