Add duk_{get,set}_finalizer C API call
API testcase for duk_{get,set}_finalizer
Add duk_{get,set}_finalizer() reference to guide
Add API docs for duk_{get,set}_finalizer()
Improve C finalizer API test case
Test case for finalizer reference loop
Release note: duk_{get,set}_finalizer()
Release note rewording
Merge branch 'api-get-set-finalizer'

Add duk_get_finalizer() and duk_set_finalizer() Duktape API calls to allow
user code to get/set finalizer from C code.  This is a common need when e.g.
Duktape/C constructors create native objects with finalizers.  It may be
awkward to jump into Ecmascript for setting up the finalizer if all the
other code is C.