Implement our OOM-handling SecondaryMap from scratch

I realized we need to adjust its `V: Clone` bound into `V: TryClone` which means
that we can no longer actually just wrap an inner
`cranelift_entity::SecondaryMap<K, V>` and need to instead implement our
own. This also made me realize that we need `remove` to be fallible because,
when the entry being removed is in bounds, it overwrites the entry with the
default value, but that default value needs to be `TryClone`d now which is, of
course, a fallible operation.

Author: fitzgen