rust: Support make dist -> cargo vendor

What we do here basically is set things up in a `dist-hook` so that our Rust
sources are vendored at `dist` time. This gives us a single tarball still, and
ideally should be transparent to downstream builders, as long as they have the
`cargo/rust` toolchain.

Closes: #669
Approved by: jlebon

rust/cargo-vendor-config

@@ -0,0 +1,8 @@
+# This is used after `cargo vendor` is run from `make dist` 
+
+[source.crates-io]
+registry = 'https://github.com/rust-lang/crates.io-index'
+replace-with = 'vendored-sources'
+
+[source.vendored-sources]
+directory = './vendor'