maint: Add script for vendoring/updating Rust dependencies.

* ADD documentation for usage both inside the script and in
   doc/HACKING/CodingStandardsRust.md
 * FIXES part of #25310: https://bugs.torproject.org/25310

doc/HACKING/CodingStandardsRust.md

@@ -81,6 +81,26 @@ Currently, Tor requires that you use the latest stable Rust version. At
 some point in the future, we will freeze on a given stable Rust version,
 to ensure backward compatibility with stable distributions that ship it.
 
+ Updating/Adding Dependencies
+------------------------------
+
+To add/remove/update dependencies, first add your dependencies,
+exactly specifying their versions, into the appropriate *crate-level*
+`Cargo.toml` in `src/rust/` (i.e. *not* `/src/rust/Cargo.toml`, but
+instead the one for your crate).  Also, investigate whether your
+dependency has any optional dependencies which are unnecessary but are
+enabled by default.  If so, you'll likely be able to enable/disable
+them via some feature, e.g.:
+
+```toml
+[dependencies]
+foo = { version = "1.0.0", default-features = false }
+```
+
+Next, run `/scripts/maint/updateRustDependencies.sh`.  Then, go into
+`src/ext/rust` and commit the changes to the `tor-rust-dependencies`
+repo.
+
  Documentation
 ---------------
 

scripts/maint/updateRustDependencies.sh

@@ -0,0 +1,45 @@
+#!/usr/bin/env bash
+#
+# Copyright (c) 2018 The Tor Project, Inc.
+# Copyright (c) 2018 isis agora lovecruft
+# See LICENSE for license information
+#
+# updateRustDependencies.sh
+# -------------------------
+# Update our vendored Rust dependencies, either adding/removing
+# dependencies and/or upgrading current dependencies to newer
+# versions.
+#
+# To use this script, first add your dependencies, exactly specifying
+# their versions, into the appropriate *crate-level* Cargo.toml in
+# src/rust/ (i.e. *not* /src/rust/Cargo.toml, but instead the one for
+# your crate).
+#
+# Next, run this script.  Then, go into src/ext/rust and commit the
+# changes to the tor-rust-dependencies repo.
+
+set -e
+
+HERE=`dirname $(realpath $0)`
+TOPLEVEL=`dirname $(dirname $HERE)`
+TOML="$TOPLEVEL/src/rust/Cargo.toml"
+VENDORED="$TOPLEVEL/src/ext/rust/crates"
+CARGO=`which cargo`
+
+if ! test -f "$TOML"  ; then
+    printf "Error: Couldn't find workspace Cargo.toml in expected location: %s\n" "$TOML"
+fi
+
+if ! test -d "$VENDORED" ; then
+    printf "Error: Couldn't find directory for Rust dependencies! Expected location: %s\n" "$VENDORED"
+fi
+
+if test -z "$CARGO" ; then
+    printf "Error: cargo must be installed and in your \$PATH\n"
+fi
+
+if test -z `cargo --list | grep vendor` ; then
+    printf "Error: cargo-vendor not installed\n"
+fi
+
+$CARGO vendor -v --locked --explicit-version --no-delete --sync $TOML $VENDORED