TinyRTC

I now have a scheme program that sets and gets the time from a TinyRTC connected to the STM32F4 discovery board. It's actually fairly neat. See examples/stm32f4-discovery-flash.scm and tinyrtc.scm. I couldn't get it to work for ages, because I'd forgotten I needed to configure the pins in open drain mode, as well as configuring I2C mode. I'll have to go back and hack on the generated use-pin code so it does that automatically. That's pretty exciting though. Using map to convert lists from binary to BCD is a neat demonstration of the relevance of scheme, as is the fact that I'll eventually be able to automate pin configuration with some simple rules.

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