Rework rustc driver about marker HOT 1 CLOSED

The structure first implemented as part of this issue has already improved the conversion process. However, after adding further nodes to the API and adding more conversion code, some limitations came up. For this reason, I'll once again try to restructure the driver code. The goal is to:

• Have consistent naming conventions (Currently, there are functions similar functions starting with different prefixes to_, to_api, conv_)
• Avoid passing cx around everywhere, I'll most likely use a single conversion struct for everything
• Define how the translation layer should usually be structure
• Cleanup lifetimes, mostly avoiding 'hir if it's not needed
• Change location of the Storage struct
• Cut clear boundaries, where conversion functions should be located

And probably other things I can't think of, rn. This comment is basically a new start for this issue.

Small side note, for those who are interested. I think one reason, why the structure is messy, is that I have no reference project. I know that rustc has a similar component to lower the AST to HIR. However, I also find that code messy. This trial and error, is not a bad thing as it makes me learn and enables me to reason, why something is better than the other. But it sadly takes time, which could be spent on expanding the API more

from marker.