Update from Rust 2018 -> Rust 2021 Edition (#32)

Author: cwaldren-ld

contract-tests/Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 name = "contract-tests"
 version = "0.1.0"
-edition = "2018"
+edition = "2021"
 license = "Apache-2.0"
 
 [dependencies]

eventsource-client/Cargo.toml

@@ -4,7 +4,7 @@ version = "0.10.0"
 description = "Client for the Server-Sent Events protocol (aka EventSource)"
 repository = "https://github.com/launchdarkly/rust-eventsource-client"
 authors = ["LaunchDarkly"]
-edition = "2018"
+edition = "2021"
 license = "Apache-2.0"
 keywords = ["launchdarkly", "feature-flags", "feature-toggles", "eventsource", "server-sent-events"]
 exclude = [