Make MemHistory::default() be a wrapper around MemHistory::new() to avoid that max_len is defaulted to 0.

[tage64]

I found that when creating MemHistory from Default::default(), the max_len (that is the maximum size of the history) is initialized to 0 due to the #[derive(Default)] on the MemHistory struct.
This is counter intuative since the history doesn't work as a history at all in that case.

I changed the Default implementation to call MemHistory::new() instead. Then the history max length will be initialized to 100 instead.

[gwenn]

Thanks.