Introduce card10-sys.
This should fix the problems encountered in !9 (closed).
The main issue there seems to be that the crates.io version is not able to access the content of the firmware even if the directory is listed inside the includes attribute of the manifest file. This is most likely related to the fact that the relative path points towards the parent directory and not a sub directory.
The fix would be to move the c sub-module into the l0dable crate. As a part of this merge request also splits the crate into two independent ones as it allows to update the API and the HAL independently.
Next to this: I am still not fully sure if the handling for l0dable.ld, crt.s and the Reset_Handler function should be part of card10-sys or card10-l0dable. Right now I moved them all over to the sys package as it allowed to move the whole build script from one crate into the other.