types REFACTOR move identityref-specific functions into a standalone source file

Split the plugins_types.c file to distinct internal implementation of
plugins types API and the (base) type plugins themselves.
diff --git a/CMakeLists.txt b/CMakeLists.txt
index f6b4b57..0d05dc4 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -140,6 +140,7 @@
     src/plugins_types_decimal64.c
     src/plugins_types_empty.c
     src/plugins_types_enumeration.c
+    src/plugins_types_identityref.c
     src/plugins_types_integer.c
     src/plugins_types_string.c
     src/plugins_exts.c