Implement more petgraph traits

[lmondada]

I noticed that portgraph's traits (PortView, LinkView) are implemented for graph objects themselves, whereas most petgraph traits (at least the ones I am using) are implemented for graph references.

Is there anything that prevents us from defining

impl<'a, G: PortView> PortView for &'a G { ... }

within portgraph?

Whilst we are at it, are the following impls (among other) also possible?

impl<G: PortView> GraphBase for G { 
    type NodeId = NodeIndex;
    ...
}
impl<'a, G: PortView> IntoNodeIndices for &'a G { ... } 
...

Such impls would greatly simplify my generics and bounds.

I might not too confident yet about which trait impls are exactly allowed in Rust, so it might be that some of those are invalid.

EDIT: it might be slightly weird if e.g. IntoNodeIndices is defined both for graphs themselves and their references, as calling g.node_identifiers() would consume g in the absence of explicit casting... I changed the second code block to only implement IntoNodeIndices for refs.

[lmondada]

From what I've tried, it seems the first code block is possible, the second is not. I'll make a draft PR.