Remove the features from the cargo build and add `inspect`
To keep things simple and predictable, let's always build all the source files with cargo. The feature selection then merely limits which files we install.
Then clean up and add the inspect
command to print data about a bpf.o file.
Closes #9 (closed) (though it doesn't have all features listed there as example)
This is partially a backup MR, happy to change but not until next week.