Convert return value of Eval and Resumable trait to SmallVec of default size 1

[maplant]

Scheme allows for the returning of multiple values for an expression, and thus we must support that. However, the vast majority of the time only one value will be returned. Therefore, we can optimize the Eval and Resumable traits to return a small vec of size 1:

pub trait Eval {
    async fn eval(
        &self,
        env: &Env,
        cont: &Option<Arc<Continuation>>,
    ) -> Result<SmallVec<[Gc<Value>; 1]>, RuntimeError>
}