backwards compatible enhancements to some embot::core data structures.

[marcoaccame]

This PR does two small backwards compatible changes:

• cleans up the API of embot::core::Data which is used inside embot as a ... generic data holder, so that now it can also manage constexpr data.

  static constexpr uint8_t bytes2write[8] {1, 2, 3, 4, 5, 6, 7, 8};
  constexpr embot::core::Data data2write {bytes2write, sizeof(bytes2write)};
  ...
  // step 03: write some data to EEPROM
  if(true == chipM95512DF->write(adr, data2write))
  ...

• adds a comment on embot::core::Callback about how to construct it w/ a lambda

  uint32_t counter {0};
  void *arg = &val;
  embot::core::Callback cbk {[](void *p){ uint32_t *v = reinterpret_cast<uint32_t*>p); *p++; }, arg};
  cbk.execute();
  // now counter has value = 1;