add update-rust-hashes.sh script

the cross-compilation process in the rust komodo recipe
differs slightly from the current native_rust recipe used
in Zcash. therefore, we have commented out all invocations
of the "update_stdlib_hash" function until the komodo rust
recipe is updated.

contrib/devtools/update-rust-hashes.sh

@@ -0,0 +1,38 @@
+#!/usr/bin/env bash
+
+export LC_ALL=C
+
+SCRIPT_DIR=$( cd -- "$( dirname -- "${BASH_SOURCE[0]}" )" &> /dev/null && pwd )
+RUST_PACKAGE="$SCRIPT_DIR/../../depends/packages/rust.mk"
+
+RUST_VERSION=$( grep -oP "_version=\K.*" $RUST_PACKAGE )
+
+update_hash() {
+    url="https://static.rust-lang.org/dist/$1-$RUST_VERSION-$2.tar.gz"
+    echo "Fetching $url"
+    hash=$( curl $url | sha256sum | awk '{print $1}' )
+    sed -i "/\$(package)_$3_$4=/c\\\$(package)_$3_$4=$hash" $RUST_PACKAGE
+}
+
+update_rust_hash() {
+    update_hash rust $1 sha256_hash $2
+}
+
+update_stdlib_hash() {
+    update_hash rust-std $1 rust_std_sha256_hash $1
+}
+
+# For native targets
+# update_rust_hash RUST_TARGET MAKEFILE_PACKAGE_IDENTIFIER
+update_rust_hash aarch64-unknown-linux-gnu aarch64_linux
+update_rust_hash x86_64-apple-darwin darwin
+update_rust_hash x86_64-unknown-linux-gnu linux
+update_rust_hash x86_64-unknown-freebsd freebsd
+update_rust_hash x86_64-pc-windows-gnu mingw32
+
+# For cross-compilation targets
+# update_stdlib_hash RUST_TARGET
+# update_stdlib_hash aarch64-unknown-linux-gnu
+# update_stdlib_hash x86_64-apple-darwin
+# update_stdlib_hash x86_64-pc-windows-gnu
+# update_stdlib_hash x86_64-unknown-freebsd