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/tools/rust/BUILD b/tools/rust/BUILD
index fccc3e4..4d9e4d6 100644
--- a/tools/rust/BUILD
+++ b/tools/rust/BUILD
@@ -132,3 +132,9 @@
     ],
 )
 '''
+
+cc_library(
+    name = "forward_allocator",
+    srcs = ["forward_allocator.c"],
+    visibility = ["//visibility:public"],
+)