Rather than fixing the version separately for each test, we can just
request to use a nightly in the `options.yml` file, with the specific
version hard-coded in `qltest.rs`. We can update it if we need to.
It's better to have a single nightly version for all tests that require
it, in order to avoid downloading more versions than necessary.
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.
For the moment this removes the possibility to set custom flags or env
variables for the extractor via comments, but that can be added back
easily if needed with a different mechanism (like an `env` file).
