1.99 beta crater regression: "type annotations needed"

This regression was found by the 1.99 crater run.

[INFO] [stdout] error[E0282]: type annotations needed for `&_`
[INFO] [stdout]    --> src/sync/mcast.rs:172:65
[INFO] [stdout]     |
[INFO] [stdout] 172 |     let reader = MpMcastReader::new(DynRef::new(queue.clone(), |q| q.as_ref()));
[INFO] [stdout]     |                                                                 ^  - type must be known at this point
[INFO] [stdout]     |
[INFO] [stdout] help: consider giving this closure parameter an explicit type, where the type for type parameter `Q` is specified
[INFO] [stdout]     |
[INFO] [stdout] 172 |     let reader = MpMcastReader::new(DynRef::new(queue.clone(), |q: &Q| q.as_ref()));
[INFO] [stdout]     |                                                                  ++++
[INFO] [stdout] 
[INFO] [stdout] 
[INFO] [stdout] For more information about this error, try `rustc --explain E0282`.

Affects the Banyc/primitive crate, and its 7 dependents, all by the same author.

• https://crater-reports.s3.amazonaws.com/beta-1.99-4/1.99.0-beta.1/gh/Banyc.primitive/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.99-4/1.99.0-beta.1/gh/Banyc.auth/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.99-4/1.99.0-beta.1/gh/Banyc.git_runner/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.99-4/1.99.0-beta.1/gh/Banyc.math/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.99-4/1.99.0-beta.1/gh/Banyc.metrics/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.99-4/1.99.0-beta.1/gh/Banyc.monitor_table/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.99-4/1.99.0-beta.1/gh/Banyc.mux/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.99-4/1.99.0-beta.1/gh/Banyc.udp_listener/log.txt

Start with the crater error at src/sync/mcast.rs:172 in the Banyc/primitive crate and compare the linked logs from its seven dependents. Reproduce the type-inference failure under Rust 1.99 beta, then trace why the DynRef::new closure no longer infers its parameter type. Done means Banyc/primitive and the listed dependents compile successfully again.