value_of metaprogram

[rtrangucci]

src/stan/agrad/fwd/functions/value_of.hpp definition:

template<typename T>
inline T value_of(const fvar<T>& v) {
  return v.val_;
}

should be updated to handle arbitrarily nested fvars for higher-order autodiff.

It may need to be another function if we're assuming value_of returns type T in some fvar<T> cases.