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.