feat(body): add body::aggregate and body::to_bytes functions

Adds utility functions to `hyper::body` to help asynchronously
collecting all the buffers of some `HttpBody` into one.

- `aggregate` will collect all into an `impl Buf` without copying the
  contents. This is ideal if you don't need a contiguous buffer.
- `to_bytes` will copy all the data into a single contiguous `Bytes`
  buffer.

src/body/to_bytes.rs

@@ -0,0 +1,36 @@
+use bytes::{Buf, BufMut, Bytes};
+
+use super::HttpBody;
+
+/// dox
+pub async fn to_bytes<T>(body: T) -> Result<Bytes, T::Error>
+where
+    T: HttpBody,
+{
+    futures_util::pin_mut!(body);
+
+    // If there's only 1 chunk, we can just return Buf::to_bytes()
+    let mut first = if let Some(buf) = body.data().await {
+        buf?
+    } else {
+        return Ok(Bytes::new());
+    };
+
+    let second = if let Some(buf) = body.data().await {
+        buf?
+    } else {
+        return Ok(first.to_bytes());
+    };
+
+    // With more than 1 buf, we gotta flatten into a Vec first.
+    let cap = first.remaining() + second.remaining() + body.size_hint().lower() as usize;
+    let mut vec = Vec::with_capacity(cap);
+    vec.put(first);
+    vec.put(second);
+
+    while let Some(buf) = body.data().await {
+        vec.put(buf?);
+    }
+
+    Ok(vec.into())
+}