gigasecond: add stub with type signature (#306)
gigasecond: add stub with type signature (#306)

The signature of the function is not at all obvious.
petertseng/exercism-rust,sacherjj/rust,sacherjj/rust,petertseng/exercism-rust,exercism/xrust,petertseng/exercism-rust,exercism/xrust,sacherjj/rust