Composable logging with monoids and contravariant functors.

If you want to use this library there are at least two ways to obtain it: you
could depend on it with Cargo by adding contralog as a dependency in
Cargo.toml, or you could copy the src/lib.rs file into your own project under
a different name.

When developing this library please use Nix to ensure the build is
reproducible:

    nix run -ic cargo test