upgrade to 2018 Rust edition

This is mostly just "cargo fix --edition" + Cargo.toml changes.
There's one fix for upgrading to NLL in db/writer.rs:
Writer::previously_opened wouldn't build with NLL because of a
double-borrow the previous borrow checker somehow didn't catch.
Restructure to avoid it.

I'll put elective NLL changes in a following commit.

src/cmds/ts.rs

@@ -28,7 +28,7 @@
 // You should have received a copy of the GNU General Public License
 // along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
-use db::recording;
+use crate::db::recording;
 use failure::Error;
 
 const USAGE: &'static str = r#"