Move i2c implementation out of macro

Author: Piroro-hs

CHANGELOG.md

@@ -51,6 +51,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
   purposes and if a new useful feature arises, we will increase the MSRV.
   ([#170](https://github.com/stm32-rs/stm32f3xx-hal/pull/170))
 - Removed I2C2 support for `stm32f303x6`, `stm32f303x8` and `stm32f328` targets. ([#164](https://github.com/stm32-rs/stm32f3xx-hal/pull/164))
+- `I2c::i2c1` and `I2c::i2c2` functions are renamed to `I2c::new`. ([#164](https://github.com/stm32-rs/stm32f3xx-hal/pull/164))
 
 ## [v0.5.0] - 2020-07-21