Annotate transpiled files with the version of Axel used to generate them, and don't allow macro imports from incompatible versions

[jgrosso]

If e.g. the macro definition semantics changes significantly between Axel versions, importing macros from old versions of Axel may cause incorrect behavior. Should we tag files with e.g. axelTranspilerVersion = "0.0.12" and then, whenever a macro is imported, import axelTranspilerVersion too and check its compatibility?

Also, what would an upgrade process look like? Presumably, "Delete the old Haskell file* and re-run Axel"?

*Maybe with e.g. axel project clean --force, see #59.