RV3028 *rtc is NULL, and some warnings

[martinwork]

RV3028 *rtc is NULL
https://github.com/sparkfun/pxt-gator-rtc/blob/master/gatorRTC.cpp#L12
A quick fix is this RV3028 theRTC; RV3028 *rtc = &theRTC;, but it might be better allocated in an init function

In the Chrome F12 Console, there is a warning about the setTime function.
/home/build/prj2/source/gatorRTC/SparkFun_RV3028.cpp: In member function 'bool RV3028::setTime(uint8_t*, uint8_t)': /home/build/prj2/source/gatorRTC/SparkFun_RV3028.cpp:111:1: warning: control reaches end of non-void function [-Wreturn-type] }
This is easily fixed by returning true.

There are a series of warnings about using mbed with CODAL
https://support.microbit.org/en/support/solutions/articles/19000054952-makecode-extension-and-tutorial-approval
"The exenstion does not make use of mBed and uses CODAL/DAL"
As far a I could see mbed was only included to get the definitions for uint8_t etc, so I think it's safe to replace it with an include of cstdint, which in also included in gatorRTC.cpp.

See
https://github.com/martinwork/pxt-gator-rtc/commits/suggestions