This collapses all generated test QL sources into a single one per
directory, using query predicates to run the different tests.
This should improve the time required to run generated tests.
This generates test source files from code blocks in class docstrings.
By default the test code is generated as is, but it can optionally:
* be wrapped in a function providing an adequate context using
`@rust.doc_test_function(name, *, lifetimes=(), return_type="()", **kwargs)`,
with `kwargs` providing both generic and normal params depending on
capitalization
* be skipped altogether using `@rust.skip_doc_test`
So for example an annotation like
```python
@rust.doc_test_function("foo",
lifetimes=("a",),
T="Eq",
x="&'a T",
y="&'a T",
return_type="&'a T")
```
will result in the following wrapper:
```rust
fn foo<'a, T: Eq>(x: &'a T, y: &'a T) -> &'a T {
// example code here
}
```
