Provide stand-alone operation
Use case is to generate wrapper once and use the generated file instead of running nimterop on every compile / requiring end user of library to have nimterop. This keeps the dependency tree of projects using wrappers a lot more manageable (no rust etc needed).

See https://github.com/arnetheduck/nim-sqlite3-abi/blob/master/update.sh#L23 for an example of this kind of setup - here are some things that would help:

* [x] avoid relying on code reordering / experimental features (wrappers are typically dumb anyway and this helps supporting older nims)
* [x] avoid importing `nimterop/types`, expanding whatever's in there into the wrapper
* [x] avoid absolute paths
* [x] possibly: avoid generating header directives completely
