upgrade to hyper 0.13 ecosystem

This doesn't take much advantage of async fns so far. For example, the
with_{form,json}_body functions are still designed to be used with
future combinators when it'd be more natural to call them from async
fns now. But it's a start.

Similarly, this still uses the old version of reqwest. Small steps.

Requires Rust 1.40 now. (1.39 is a requirement of async, and 1.40 is a
requirement of http-serve 0.2.0.)

scripts/script-functions.sh

@@ -40,7 +40,7 @@ fi
 NODE_MIN_VERSION="8"
 YARN_MIN_VERSION="1.0"
 CARGO_MIN_VERSION="0.2"
-RUSTC_MIN_VERSION="1.36"
+RUSTC_MIN_VERSION="1.40"
 
 normalizeDirPath()
 {