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/mod.rs

@@ -32,7 +32,7 @@
 ///
 /// See `guide/schema.md` for more information.
 
-use db;
+use crate::db;
 use failure::Error;
 use rusqlite::{self, types::ToSql};
 

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;
 

db/upgrade/v1_to_v2.rs

@@ -30,11 +30,11 @@
 
 /// Upgrades a version 1 schema to a version 2 schema.
 
-use dir;
+use crate::dir;
 use failure::Error;
 use libc;
 use rusqlite::{self, types::ToSql};
-use schema::DirMeta;
+use crate::schema::DirMeta;
 use std::fs;
 use std::os::unix::ffi::OsStrExt;
 use uuid::Uuid;
@@ -332,7 +332,7 @@ fn verify_dir_contents(sample_file_path: &str, tx: &rusqlite::Transaction) -> Re
         let mut rows = stmt.query(&[] as &[&ToSql])?;
         while let Some(row) = rows.next() {
             let row = row?;
-            let uuid: ::db::FromSqlUuid = row.get_checked(0)?;
+            let uuid: crate::db::FromSqlUuid = row.get_checked(0)?;
             if !files.remove(&uuid.0) {
                 bail!("{} is missing from dir {}!", uuid.0, sample_file_path);
             }
@@ -343,7 +343,7 @@ fn verify_dir_contents(sample_file_path: &str, tx: &rusqlite::Transaction) -> Re
     let mut rows = stmt.query(&[] as &[&ToSql])?;
     while let Some(row) = rows.next() {
         let row = row?;
-        let uuid: ::db::FromSqlUuid = row.get_checked(0)?;
+        let uuid: crate::db::FromSqlUuid = row.get_checked(0)?;
         files.remove(&uuid.0);
     }
 

db/upgrade/v2_to_v3.rs

@@ -32,11 +32,11 @@
 /// Note that a version 2 schema is never actually used; so we know the upgrade from version 1 was
 /// completed, and possibly an upgrade from 2 to 3 is half-finished.
 
-use db::{self, FromSqlUuid};
-use dir;
+use crate::db::{self, FromSqlUuid};
+use crate::dir;
 use failure::Error;
 use libc;
-use schema;
+use crate::schema;
 use std::io::{self, Write};
 use std::mem;
 use std::sync::Arc;