* Use the new protobuf messages that send accounts as strings (fixes#367)
* `stellar_signingAbort()` now takes an error message
* Operations now check if a transaction is being signed (fixes#368)
* Operations now return false on error or if the user cancels
* Stellar `fsm_*` methods now check operation result and return early if the operation fails
This PR also re-enables the stellar code in the firmware.
