Rust: add proc-macro capabilities to QL tests

This adds the possibility to add a special `proc_macro.rs` source file
to QL tests, which will be generated into a `proc_macro` crate the
usual `lib` crate depends on.

This allow to define procedural macros in QL tests, and is here used to
move the `macro-expansion` integration test to be a language test
instead.

As the generated manifests involved were starting to get a bit complex,
they are now generated from a `mustache` template.

Cargo.lock

@@ -426,6 +426,7 @@ dependencies = [
  "figment",
  "glob",
  "itertools 0.14.0",
+ "mustache",
  "num-traits",
  "ra_ap_base_db",
  "ra_ap_cfg",