Rewrite PyUnicode_Append(); unicode_modifiable() is more strict

* Rename unicode_resizable() to unicode_modifiable()
 * Rename _PyUnicode_Dirty() to unicode_check_modifiable() to make it clear
   that the function is private
 * Inline PyUnicode_Concat() and unicode_append_inplace() in PyUnicode_Append()
   to simplify the code
 * unicode_modifiable() return 0 if the hash has been computed or if the string
   is not an exact unicode string
 * Remove _PyUnicode_DIRTY(): no need to reset the hash anymore, because if the
   hash has already been computed, you cannot modify a string inplace anymore
 * PyUnicode_Concat() checks for integer overflow