* add regression test for missing Send requirement
* fix: require the initialization with `F: FnOnce` to be `Send` as well
If creating intermediate variables of the builder type within
a future, rustc will complain about the future not being send,
while the thing itself isn't even using the closure based field
initialization. Adding an additional bound, resolves this and
pushes the error message "closer" to the user, and out of the
generated code.
* import fixins
* Parse generic baggage types more carefully to preserve inner structure
* Add example
* Way too many clones
* Allow multiple generic arguments for baggage fields
* Try to detect errors earlier
* Support more types for the baggage fields, get rid of the path constraint