grade-school: Use `Entry#or_insert` in `add`
grade-school: Use `Entry#or_insert` in `add`

This drastically shortens the function, avoiding the need to `match` on
the `Entry` cases.
This seems good to make the example more idiomatic.

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