golang_str: Fix ustr.translate on sequence

NumPy uses s.translate(str) and under gpython/py3 with str patched to be
ustr it breaks with:

      File ".../numpy-1.24.4-py3.9-linux-x86_64.egg/numpy/core/_string_helpers.py", line 40, in english_lower
        lowered = s.translate(LOWER_TABLE)
      File "golang/_golang_str.pyx", line 909, in golang._golang._pyustr.translate
    AttributeError: 'str' object has no attribute 'items'

https://docs.python.org/3/library/stdtypes.html#str.translate documents
translate to work on both mappings and sequences, so my usage of
table.items() in ff24be3d (golang_str: bstr/ustr string methods) was not
correct.

-> Fix it by reworking ustr.translate to use our proxy mapping instead
of going through all items of original table in the beginning.

golang/_golang_str.pyx

@@ -986,12 +986,7 @@ cdef class _pyustr(unicode):
 
     def translate(self, table):
         # unicode.translate does not accept bstr values
-        t = {}
-        for k,v in table.items():
-            if not isinstance(v, int):  # either unicode ordinal,
-                v = _xpyu_coerce(v)     # character or None
-            t[k] = v
-        return pyu(zunicode.translate(self, t))
+        return pyu(zunicode.translate(self, _pyustrTranslateTab(table)))
 
     def upper(self):                        return pyu(zunicode.upper(self))
     def zfill(self, width):                 return pyu(zunicode.zfill(self, width))
@@ -1090,6 +1085,18 @@ def pyuiter(obj):
     return iter(pyu(obj))   # TODO iterate obj directly
 
 
+# _pyustrTranslateTab wraps table for .translate to return bstr as unicode
+# because unicode.translate does not accept bstr values.
+cdef class _pyustrTranslateTab:
+    cdef object tab
+    def __init__(self, tab):
+        self.tab = tab
+    def __getitem__(self, k):
+        v = self.tab[k]
+        if not isinstance(v, int):  # either unicode ordinal,
+            v = _xpyu_coerce(v)     # character or None
+        return v
+
 
 # _bdata/_udata retrieve raw data from bytes/unicode.
 def _bdata(obj): # -> bytes

golang/golang_str_test.py

@@ -1886,6 +1886,7 @@ def test_strings_methods():
     _("МиР").swapcase(                              ok="мИр")
     _("МиР").title(                                 ok="Мир")
     _("мир").translate({ord(u'м'):ord(u'и'), ord(u'и'):'я', ord(u'р'):None},        ok="ия")
+    _(u"\u0000\u0001\u0002.").translate([u'м', ord(u'и'), None],                    ok="ми.")
     _("МиР").upper(                                 ok="МИР")
     _("мир").zfill(10,                              ok="0000000мир")
     _("123").zfill(10,                              ok="0000000123")