fix dead code analysis warning

[cosmicexplorer]

#79 is failing CI due to a warning introduced in src/read/stream.rs:

 error: struct `CounterVisitor` is never constructed
Error:    --> src/read/stream.rs:224:12
    |
224 |     struct CounterVisitor(u64, u64);
    |            ^^^^^^^^^^^^^^
    |
    = note: `CounterVisitor` has a derived impl for the trait `Default`, but this is intentionally ignored during dead code analysis
    = note: `-D dead-code` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(dead_code)]`

This appears to have been introduced by a newer nightly version updating behind our back and introducing this error, and not by any recent code change. I believe this should fix it.

We may be able to delete CounterVisitor entirely, but I'm not familiar with the testing in stream.rs so decided to avoid making bigger changes.