Add preliminary support for some STM32G0 chips. (closes #759)

[WRansohoff]

This is just a starting point and I've only tested it with a small 'hello world' example on one STM32G071GB board, but I ordered one of ST's Nucleo boards now that they're back in stock, so hopefully I can try it with some STCube examples before long.

TODO:

• Use the MER 'mass erase' option for erasing flash instead of going page-by-page.
• The STM32G0 reference manual mentions a 'fast programming' mode which I haven't looked at.
• It would probably be easier to read if I updated and used the methods like 'wait_flash_busy', 'unlock_flash', etc.
• Some of the flash register definitions are not used.

[WRansohoff]

I found a bug in the flash-erasing section which caused some pages to not be erased; sorry. I think it works now, though.

[xor-gate]

Thanks!