Allow calling bindings from views when generating C stubs

Usually you are not allowed to call the just defined bindings when
generating C stubs (because they are not real functions yet).
For example the call to 'error ()' should be replaced by '(view_invoke error)
()' here: https://github.com/dbuenzli/tsdl/blob/bc18d14b1a28af99ebc30b61e303ee5304055d31/src/tsdl.ml#L82-L89