[Rust APIView] Impl block is missing generic parameter in rendering

Bad code snippet:
```rust
impl FeedItemIterator {
```

What it should do:
The APIView rendering should include the generic parameter on the impl block, since the type is actually generic. This appears to be a parser/rendering bug rather than a code suggestion issue.

Good code snippet:
```rust
impl FeedItemIterator {
```

Start with the Rust APIView parser and rendering path responsible for impl blocks, using the bad and good snippets in this issue as the reproduction. Confirm that the generic parameter and generic type are preserved in the rendered output; done means the impl block renders as `impl FeedItemIterator`.

Written by the indexing model from the issue text.