Adding a new io vector support function; iovec_size().
Reducing IOStream footprint; removing IOStream::Filter and null device.
Improving SPI; bit_set to bit_mask_set, new member function for iovec_t write.
Reducing RTC::Timer footprint; refactoring to new class Timer.

1. Stepping back to RTC interrupt handler extension.
2. Adding Timer interrupt handling as an extension.
3. Adding a new Timer member function to register extension.
4. Updating benchmark sketch.