parallel-letter-frequency: Use `Entry#or_insert`
parallel-letter-frequency: Use `Entry#or_insert`

This drastically shortens the `count` function and result collection
loop in `frequency`, avoiding the need to `match` on the `Entry` cases.
This seems good to make the example more idiomatic.

sacherjj/rust,exercism/xrust,exercism/xrust,petertseng/exercism-rust,sacherjj/rust,petertseng/exercism-rust,sacherjj/rust,petertseng/exercism-rust