Remove lib.rs.

Remove lib.rs.

I'm a bit concerned that with a main.rs and a lib.rs, this package
might accidentally have made both a binary and a library crate,
which (as and when I publish it) might cause people to try to use the
library as if it had a stable and committed API.

In fact all the library modules in this source base are considered
internal to the application, and the interfaces between them are
intended to vary however this application finds convenient. So I
shouldn't accidentally imply otherwise.