Support linking Rust code into a C++ binary

This is based on a version of my upstream PR:
https://github.com/bazelbuild/rules_rust/pull/1350

Change-Id: Ica7097c10666d2e0017336cf7d81420605238493
Signed-off-by: Brian Silverman <bsilver16384@gmail.com>
diff --git a/WORKSPACE b/WORKSPACE
index 50aad8c..79af5a3 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -841,6 +841,7 @@
 
 rust_toolchain_repository(
     name = "rust",
+    allocator_library = "@//tools/rust:forward_allocator",
     edition = "2021",
     exec_triple = "x86_64-unknown-linux-gnu",
     extra_target_triples = [