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.

db/upgrade/v0_to_v1.rs

@@ -30,9 +30,9 @@
 
 /// Upgrades a version 0 schema to a version 1 schema.
 
-use db;
+use crate::db;
 use failure::Error;
-use recording;
+use crate::recording;
 use rusqlite::{self, types::ToSql};
 use std::collections::HashMap;