fixed warning 'trait objects without an explicit are deprecated'

[lorlouis]

Rust is moving towards using explicit &dyn pointer in order to differentiate them from & pointer which can only be used to reference a struct.

see https://doc.rust-lang.org/1.30.0/book/first-edition/trait-objects.html#static-dispatch

https://doc.rust-lang.org/1.30.0/book/first-edition/trait-objects.html#dynamic-dispatch