Fixed compiler warnings as directed by compiler hints.

[petertrotman]

cargo 1.38.0 (23ef9a4ef 2019-08-20)
rustc 1.38.0 (625451e37 2019-09-23)

Previously:

warning: trait objects without an explicit `dyn` are deprecated
  --> src/cmd/headers.rs:60:22
   |
60 |     let mut wtr: Box<io::Write> =
   |                      ^^^^^^^^^ help: use `dyn`: `dyn io::Write`
   |
   = note: `#[warn(bare_trait_objects)]` on by default

warning: trait objects without an explicit `dyn` are deprecated
   --> src/cmd/join.rs:281:44
    |
281 |         -> CliResult<IoState<fs::File, Box<io::Write+'static>>> {
    |                                            ^^^^^^^^^^^^^^^^^ help: use `dyn`: `dyn io::Write+'static`

warning: trait objects without an explicit `dyn` are deprecated
   --> src/cmd/partition.rs:138:36
    |
138 | type BoxedWriter = csv::Writer<Box<io::Write+'static>>;
    |                                    ^^^^^^^^^^^^^^^^^ help: use `dyn`: `dyn io::Write+'static`

warning: trait objects without an explicit `dyn` are deprecated
   --> src/cmd/split.rs:131:36
    |
131 |     ) -> CliResult<csv::Writer<Box<io::Write+'static>>> {
    |                                    ^^^^^^^^^^^^^^^^^ help: use `dyn`: `dyn io::Write+'static`

warning: trait objects without an explicit `dyn` are deprecated
   --> src/config.rs:197:48
    |
197 |                  -> io::Result<csv::Writer<Box<io::Write+'static>>> {
    |                                                ^^^^^^^^^^^^^^^^^ help: use `dyn`: `dyn io::Write+'static`

warning: trait objects without an explicit `dyn` are deprecated
   --> src/config.rs:202:48
    |
202 |                  -> io::Result<csv::Reader<Box<io::Read+'static>>> {
    |                                                ^^^^^^^^^^^^^^^^ help: use `dyn`: `dyn io::Read+'static`

warning: trait objects without an explicit `dyn` are deprecated
   --> src/config.rs:263:47
    |
263 |     pub fn io_reader(&self) -> io::Result<Box<io::Read+'static>> {
    |                                               ^^^^^^^^^^^^^^^^ help: use `dyn`: `dyn io::Read+'static`

warning: trait objects without an explicit `dyn` are deprecated
   --> src/config.rs:293:47
    |
293 |     pub fn io_writer(&self) -> io::Result<Box<io::Write+'static>> {
    |                                               ^^^^^^^^^^^^^^^^^ help: use `dyn`: `dyn io::Write+'static`

warning: trait objects without an explicit `dyn` are deprecated
   --> src/util.rs:190:48
    |
190 |                  -> io::Result<csv::Writer<Box<io::Write+'static>>>
    |                                                ^^^^^^^^^^^^^^^^^ help: use `dyn`: `dyn io::Write+'static`

warning: trait objects without an explicit `dyn` are deprecated
   --> tests/tests.rs:105:29
    |
105 |     fn shrink(&self) -> Box<Iterator<Item=CsvRecord>+'static> {
    |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `dyn`: `dyn Iterator<Item=CsvRecord>+'static`
    |
    = note: `#[warn(bare_trait_objects)]` on by default

warning: trait objects without an explicit `dyn` are deprecated
   --> tests/tests.rs:151:29
    |
151 |     fn shrink(&self) -> Box<Iterator<Item=CsvData>+'static> {
    |                             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `dyn`: `dyn Iterator<Item=CsvData>+'static`

warning: use of deprecated item 'std::sync::atomic::ATOMIC_USIZE_INIT': the `new` function is now preferred
  --> tests/workdir.rs:17:39
   |
17 | static NEXT_ID: atomic::AtomicUsize = atomic::ATOMIC_USIZE_INIT;
   |                                       ^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace the use of the deprecated item: `AtomicUsize::new(0)`
   |
   = note: `#[warn(deprecated)]` on by default