Add duk_{get,require,push}_heapptr() API calls
duk_{get,require,push}_heapptr() API tests
duk_{get,require,push}_heapptr() API docs
Release note: duk_get_heapptr() and friends
Merge branch 'api-get-heapptr'

Add API calls duk_get_heapptr() and duk_require_heapptr() to get a borrowed
void pointer reference to Duktape heap allocated values (objects, strings,
and buffers).

Add API call duk_push_heapptr() to recreate an object reference based on a
borrowed pointer later.  However, the original value must have been reachable
all the way to this call, so it still needs to be stashed as before.