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/check.rs

@@ -30,14 +30,14 @@
 
 //! Subcommand to check the database and sample file dir for errors.
 
-use db::{self, CompositeId, FromSqlUuid};
-use dir;
+use crate::db::{self, CompositeId, FromSqlUuid};
+use crate::dir;
 use failure::Error;
 use fnv::FnvHashMap;
-use raw;
-use recording;
+use crate::raw;
+use crate::recording;
 use rusqlite::{self, types::ToSql};
-use schema;
+use crate::schema;
 use std::os::unix::ffi::OsStrExt;
 use std::fs;