libgolang: Provide string as alias for std::string

std::string is frequently too cumbersome. Providing string in golang::
namespace allows to have a more closer feel of a Go environment after
`using namespace golang`.

golang/_errors.pxd

@@ -24,8 +24,7 @@
 See also https://golang.org/pkg/errors for Go errors package documentation.
 """
 
-from golang cimport error
-from libcpp.string cimport string
+from golang cimport error, string
 
 cdef extern from "golang/errors.h" namespace "golang::errors" nogil:
     error New(const string& text)

golang/_golang.pxd

@@ -46,7 +46,6 @@ In addition to Cython/nogil API, golang.pyx provides runtime for golang.py:
 
 from libcpp cimport nullptr_t, nullptr as nil
 from libcpp.utility cimport pair
-from libcpp.string  cimport string
 from libc.stdint cimport uint64_t
 cdef extern from *:
     ctypedef bint cbool "bool"
@@ -61,6 +60,7 @@ cdef extern from *:
 #
 # -> golang.pyx users need to add `except +topyexc` to their functions that are
 # on the edge of Python/nogil world.
+from libcpp.string cimport string  # golang::string = std::string
 cdef extern from "golang/libgolang.h" namespace "golang" nogil:
     void panic(const char *)
     const char *recover()

golang/errors.cpp

@@ -21,7 +21,6 @@
 // See errors.h for package overview.
 
 #include "golang/errors.h"
-using std::string;
 
 
 // golang::errors::

golang/errors.h

@@ -33,7 +33,7 @@ namespace golang {
 namespace errors {
 
 // New creates new error with provided text.
-LIBGOLANG_API error New(const std::string& text);
+LIBGOLANG_API error New(const string& text);
 
 }}  // golang::errors::
 

golang/libgolang.h

@@ -298,12 +298,16 @@ LIBGOLANG_API extern void (*_tblockforever)(void);
 #include <functional>
 #include <initializer_list>
 #include <memory>
+#include <string>
 #include <type_traits>
 #include <utility>
 #include <vector>
 
 namespace golang {
 
+// string is alias for std::string.
+using string = std::string;
+
 // func is alias for std::function.
 template<typename F>
 using func = std::function<F>;
@@ -713,7 +717,7 @@ typedef refptr<_interface> interface;
 
 // error is the interface describing errors.
 struct _error : _interface {
-    virtual std::string Error() = 0;
+    virtual string Error() = 0;
 };
 typedef refptr<_error> error;
 

golang/pyx/runtime.h

@@ -65,7 +65,7 @@ public:
     LIBPYXRUNTIME_API void decref();
 
     // error interface
-    std::string Error();
+    string Error();
 
 private:
     _PyError(const _PyError&);  // don't copy

golang/pyx/runtime.pxd

@@ -25,9 +25,8 @@ Python/Cython runtimes that can be used from nogil code.
  - `PyFunc` represents Python function that can be called from nogil code.
 """
 
-from golang  cimport error, _error, refptr, gobject
+from golang  cimport error, _error, refptr, gobject, string
 from cpython cimport PyObject
-from libcpp.string  cimport string
 
 
 cdef extern from "golang/pyx/runtime.h" namespace "golang::pyx::runtime" nogil:

golang/runtime/libgolang.cpp

@@ -54,7 +54,6 @@ using std::bad_alloc;
 using std::exception;
 using std::max;
 using std::numeric_limits;
-using std::string;
 using std::unique_ptr;
 using std::vector;
 

golang/runtime/libpyxruntime.cpp

@@ -26,7 +26,6 @@
 #include "golang/sync.h"
 #include "golang/errors.h"
 using namespace golang;
-using std::string;
 
 #include <utility>
 using std::tuple;