Reference to HugrView is not a HugrView

[acl-cqc]

All(?) the methods in HugrView take &self, so to use it, h: &impl HugrView is all you need - easy for functions. However, for structs/storage, you have to decide between MyStruct<H:HugrView>{ h: H .... }, or MyStruct<'a, H:HugrView> { h: &'a H ... }, which is awkward and inflexible.

In practice this will probably mean replacing the current impl<T:AsRef<Hugr>> HugrView for T with two impls:

impl HugrView for Hugr { ... }
impl<T: HugrView> HugrView for &T

(possibly three, for &mut T, if we extend this to HugrMut, which we may want to too, although perhaps less important). Unfortunately this means more code/duplication, but the current situation is awkward for (increasing numbers of) clients....