[mcspr]

ref. #5922 for the original .ld snippet
add the missing member function pointer to invoker aka actual func obj
remove everything else that sort-of matches std::function signature. w/ a simple web server app this frees up ~600bytes of IRAM.

function dispatch rewrite inlines interrupt caller operator()(), but the lambda still needs to be in iram. this happens by matching the invoker func signature, which has esp8266::FunctionalInterrupt::Tag in its arguments list. caller may use auto, since the struct does nothing otherwise

// ...
  int foobar = 5;
  attachInterrupt(2, [&](auto) { foobar += 5; }, CHANGE);
// ...

(also choosing to ignore the throw on null func check inside operator()(). at least for now. in case there is a way to shorten the operator method w/o rewriting the header, it would be a nice addition here specifically. barring an option of a whole other lambda functor container&caller class)

SPISlave plus example rewritten to accept pointers, since sample lambdas were not actually utilizing std::function container in any way.

note that scheduled_function & scheduled interrupt sort-of work (?) under the same assumption
but, it has a whole bunch of other issues like malloc/new, constructors and copies being done all at once