The prior HTTP implementation was `hyper`, and provided a low level
of abstraction. This low level required me to implement redirect and
proxy support. I omitted proxy support.

Now, we use `reqwest` to handle HTTP. This allows us to delete our
custom (but not special) redirect support and pick up proxy support.

We have come rust compiler warnings:

```
warning: trait objects without an explicit `dyn` are deprecated
   --> pdsc/src/component.rs:225:17
    |
225 | ) -> Result<Box<Iterator<Item = ComponentBuilder>>, Error> {
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `dyn`: `dyn Iterator<Item = ComponentBuilder>`
    |
    = note: `#[warn(bare_trait_objects)]` on by default

   Compiling reqwest v0.9.22
   Compiling cmsis-update v0.1.0 (/home/jankii01/mbed/cmsis-pack-manager/rust/cmsis-update)
warning: trait objects without an explicit `dyn` are deprecated
   --> cmsis-update/src/download.rs:153:14
    |
153 |     ) -> Box<Future<Item = (), Error = Error> + 'a> {
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `dyn`: `dyn Future<Item = (), Error = Error> + 'a`
    |
    = note: `#[warn(bare_trait_objects)]` on by default

warning: trait objects without an explicit `dyn` are deprecated
   --> cmsis-update/src/download.rs:188:14
    |
188 |     ) -> Box<Stream<Item = PathBuf, Error = Error> + 'a>
    |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: use `dyn`: `dyn Stream<Item = PathBuf, Error = Error> + 'a`
```

The compiler is pretty cool, as the warning it gives already tells you
the real fix, too.

Fix trait objects without explicit `dyn`.