bpo-1635741: _PyUnicode_Name_CAPI moves to internal C API (GH-22713)

The private _PyUnicode_Name_CAPI structure of the PyCapsule API
unicodedata.ucnhash_CAPI moves to the internal C API. Moreover, the
structure gets a new state member which must be passed to the
getcode() and getname() functions.

* Move Include/ucnhash.h to Include/internal/pycore_ucnhash.h
* unicodedata module is now built with Py_BUILD_CORE_MODULE.
* unicodedata: move hashAPI variable into unicodedata_module_state.
diff --git a/Modules/Setup b/Modules/Setup
index 87f3a7c..6f9bb81 100644
--- a/Modules/Setup
+++ b/Modules/Setup
@@ -185,7 +185,7 @@
 #_json -I$(srcdir)/Include/internal -DPy_BUILD_CORE_BUILTIN _json.c	# _json speedups
 #_statistics _statisticsmodule.c # statistics accelerator
 
-#unicodedata unicodedata.c    # static Unicode character database
+#unicodedata unicodedata.c -DPy_BUILD_CORE_BUILTIN   # static Unicode character database
 
 
 # Modules with some UNIX dependencies -- on by default: