async/await, 2018 edition, updated deps

[brigand]

This brings the crate up to date with changes in the ecosystem.

The changes here backwards-incompatible, but allow it to be easily used with recent tokio (std::future based) and k8s-openapi-codegen versions (previously broken by a switch to http@2.x.x).

The file system I/O is still done with blocking APIs, as the code looks tricky to migrate. The consumer can schedule that part with spawn_blocking if needed.

Tests and the list_pod example work. I haven't tested the incluster_config example.

[brigand]

I've learned that more work is needed to properly support the newer version of k8s-openapi.

Properly making the request is more like the following, where the tuple argument is returned by the k8s-openapi functions. This has the benefit of encoding the request/response types in one function call, rather than manually specifying the expected response type.

use tokio::stream::StreamExt;
use k8s_openapi as kapi;

// ...snip

    pub async fn request<T>(
        &self,
        request: (
            http::Request<Vec<u8>>,
            fn(_: http::StatusCode) -> kapi::ResponseBody<T>,
        ),
    ) -> Result<T>
    where
        T: kapi::Response,
    {
        let (parts, body) = request.0.into_parts();
        let uri_str = format!("{}{}", self.host, parts.uri);
        let mut req = match parts.method {
            http::Method::GET => self.client.get(&uri_str),
            http::Method::POST => self.client.post(&uri_str),
            http::Method::DELETE => self.client.delete(&uri_str),
            http::Method::PUT => self.client.put(&uri_str),
            method => {
                bail!(other(format!("Unexpected HTTP method {:?}", method)));
            }
        };

        let res = req.body(body).send().await?;
        let mut k8s_res = request.1(res.status());

        let mut stream = res.bytes_stream();

        while let Some(item) = stream.next().await {
            if let Ok(item) = item {
                k8s_res.append_slice(item.as_ref());
            }
        }

        let result = k8s_res.parse()?;

        Ok(result)
    }

Currently the PR probably shouldn't be merged.