Add post_update.sh

* Include out/count.rs in src/lib.rs.
* Update src/lib.rs to match simple post_update.sh output.

Bug: 169118732
Test: tools/external_updater/updater.sh update rust/crates/proc-macro-nested
Test: mm in external/rust/crates/proc-macro-nested
Change-Id: I6fb8637205bc0742e35ef222362a459e483e4cd4
2 files changed