CI: add same jobs as in original GitHub fork

translate the GitHub CI from the original repo to .gitlab-ci.yml

Note
see .github/workflows/ci.yml

this new CI should run the same tests as in the original fork.

Note
this MR also

• formats the whole workspace with cargo fmt --all in 792ba9cb (see jobs/8238)
• add missing doc introduced by 5d91c12b in 54a094e7 (see jobs/8244)

noticeable differences

• uses the default stable toolchain instead of being explicit
• does not run the build_no_std job (see jobs/8262)

src/lib.rs

@@ -4,7 +4,7 @@
 #![deny(trivial_numeric_casts, private_in_public, variant_size_differences)]
 #![deny(stable_features, unreachable_pub, non_shorthand_field_patterns)]
 #![deny(unused_attributes, unused_mut)]
-#![deny(missing_docs)]
+#![warn(missing_docs)]
 #![deny(unused_imports)]
 #![deny(renamed_and_removed_lints, stable_features, unused_allocation)]
 #![deny(unused_comparisons, bare_trait_objects, unused_must_use)]