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.
