bpo-1635741: Add a global module state to unicodedata

[vstinner]

Prepare unicodedata to add a state per module: start with a global
"module" state, pass it to subfunctions which access &UCD_Type. This
change also prepares the conversion of the UCD_Type static type to a
heap type.

https://bugs.python.org/issue1635741

[vstinner]

With this PR, I understood that my main concern comes from the PyCapsule API: unicodedata.ucnhash_CAPI.

_PyUnicode_DecodeUnicodeEscape() uses it like this:

ucnhash_CAPI->getcode(NULL, start, (int)namelen, &ch, 0) 

No state is passed to _getcode() and so currently it can only access global variables.

I looked at how to create a C function which would be a wrapper to _getcode() which automatically pass a state. Problem: the only portable way is to use it with something like: func(closure, ...), you must pass a state to the wrapper as an argument. There is a non-portable way to really create a closure in C, but it requires libffi which sounds a heavy solution.

Hopefully we don't need to go that far. _PyUnicode_Name_CAPI is private and excluded from the limited C API. We can move it to the internal C API and introduce incompatible change, like require to pass a state. For example, we can add a state in the _PyUnicode_Name_CAPI structure and require the caller to pass it:

ucnhash_CAPI->getcode(ucnhash_CAPI->state, NULL, start, (int)namelen, &ch, 0)