Make the hello world example compile

[frankmcsherry]

Hrm. This already passes for me at the moment:

mcsherry@dataflower timely-dataflow % cargo test
running 151 tests
...
test src/lib.rs - (line 25) ... ok
...

What are you running that it produces an error?

[def-]

I see. I pasted it into a new file and ran it with cargo run.

[frankmcsherry]

Ah, I see. cargo test will add some amount of goo to doctests such as linking in the crate under test and putting a main function around what you've written. Roughly to avoid having all the boiler plate show up for all the examples in the documentation.