Rethink STM32 I2C v2 HAL (#78) ...

* Initial attempt at rethinking the STM32 I2C v2 HAL.  Makes single-byte work properly and adds a new 'state' variable to track what the hardware is doing.

* Fix some initial test failures

* Fix incorrect logic

* Fix more incorrect logic

* Tabs to spaces

* Fix repeated starts with single-byte API

* Fix race condition causing stop() after nacked address to sometimes break things

* Fix missed i2c structs that should have been removed

* Fix doing a repeated start from single-byte to transaction API causing I2C peripheral to lock up

* Fix xferOperation being set wrong for repeated starts, causing the peripheral to hang

* Fix race condition with repeated start after single-byte operation

* Fix compilation for targets that use I2C IP v1

* Fix initialization of XferOperation for API v1, optimize stop()

* Remove unneeded line

* Add docs for I2C events

Lots of usability improvements for the I2C API. Better docs and new top-level functions. (#64) ...

* Lots of usability improvements for the I2C API.  Better docs and new top-level functions.

* Document frequencies

* Tabs to spaces

* More style fixes

* Run astyle

* Clean up docs

* Add note about addressing, change 10 bit to 11 bit

* Fix spellcheck

* Fix paste error

* Oops, fix accidental change

Refactor hal directory