Add some missing functions to libcpp maps and string (GH-4395)

* add swap() to libcpp.string
* add load_factor() to libcpp.unordered_map and libcpp.unordered_set

Cython/Includes/libcpp/string.pxd

@@ -62,6 +62,7 @@ cdef extern from "<string>" namespace "std" nogil:
         void resize(size_t) except +
         void resize(size_t, char) except +
         void shrink_to_fit() except +
+        void swap(string& other)
         size_t capacity()
         void reserve(size_t) except +
         void clear()

Cython/Includes/libcpp/unordered_map.pxd

@@ -66,6 +66,7 @@ cdef extern from "<unordered_map>" namespace "std" nogil:
         #value_compare value_comp()
         void max_load_factor(float)
         float max_load_factor()
+        float load_factor()
         void rehash(size_t)
         void reserve(size_t)
         size_t bucket_count()

Cython/Includes/libcpp/unordered_set.pxd

@@ -51,6 +51,7 @@ cdef extern from "<unordered_set>" namespace "std" nogil:
         #value_compare value_comp()
         void max_load_factor(float)
         float max_load_factor()
+        float load_factor()
         void rehash(size_t)
         void reserve(size_t)
         size_t bucket_count()
@@ -110,6 +111,7 @@ cdef extern from "<unordered_set>" namespace "std" nogil:
         #value_compare value_comp()
         void max_load_factor(float)
         float max_load_factor()
+        float load_factor()
         void rehash(size_t)
         void reserve(size_t)
         size_t bucket_count()

tests/run/cpp_stl_cpp11.pyx

@@ -137,6 +137,7 @@ def test_unordered_set_functionality():
     int_set.bucket_count()
     int_set.max_bucket_count()
     int_set.bucket(3)
+    assert int_set.load_factor() > 0
     return "pass"
 
 
@@ -187,6 +188,7 @@ def test_unordered_map_functionality():
     int_map.bucket_count()
     int_map.max_bucket_count()
     int_map.bucket(3)
+    assert int_map.load_factor() > 0
 
     intptr_map[0] = NULL
     intptr = intptr_map.const_at(0)

tests/run/cpp_stl_multiset.pyx

@@ -104,3 +104,12 @@ def test_unordered_multiset_find_erase(vals, to_remove):
     it = ms.find(to_remove)
     ms.erase(it)
     return sorted([ item for item in ms ])
+
+
+def test_unordered_multiset_misc():
+    """
+    >>> test_unordered_multiset_misc()
+    """
+    cdef unordered_multiset[int] ms = unordered_multiset[int]()
+    ms.insert(1)
+    assert ms.load_factor() > 0

tests/run/cpp_stl_string.pyx

@@ -438,6 +438,16 @@ def test_stof(char *a):
     return stof(s)
 
 
+def test_swap():
+    """
+    >>> test_swap()
+    """
+    cdef string s1 = b_asdf, s_asdf = b_asdf
+    cdef string s2 = b_asdg, s_asdg = b_asdg
+    s1.swap(s2)
+    assert s1 == s_asdg and s2 == s_asdf
+
+
 _WARNINGS = """
 21:31: Cannot pass Python object as C++ data structure reference (string &), will pass by copy.
 """