Method erasure

include/function2/function2.hpp:1146–1148

  }

  FU2_DETAIL_CXX14_CONSTEXPR erasure() noexcept {
    vtable_.set_empty();
  }

  FU2_DETAIL_CXX14_CONSTEXPR erasure(std::nullptr_t) noexcept {
    vtable_.set_empty();