Rust: generate test code from schema docstrings

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
}
```

misc/codegen/generators/__init__.py

@@ -1,4 +1,4 @@
-from . import dbschemegen, qlgen, trapgen, cppgen, rustgen
+from . import dbschemegen, trapgen, cppgen, rustgen, rusttestgen, qlgen
 
 
 def generate(target, opts, renderer):