Add a script to update lock files (#379)

This can be manually run by contributors in order to keep their PRs up
to date.

Sourced [from
rust-bitcoin](https://github.com/rust-bitcoin/rust-bitcoin/blob/master/contrib/update-lock-files.sh)

contrib/update-lock-files.sh

@@ -0,0 +1,11 @@
+#!/usr/bin/env bash
+#
+# Update the minimal/recent lock file
+
+set -euo pipefail
+
+for file in Cargo-minimal.lock Cargo-recent.lock; do
+    cp -f "$file" Cargo.lock
+    cargo check
+    cp -f Cargo.lock "$file"
+done