IRI: Box the Resource(NodeId) variant to shrink the IRI enum
IRI: Box the Resource(NodeId) variant to shrink the IRI enum

`cargo +nightly rustc -- -Z print-type-sizes` gives these:

Before:
print-type-size type: `librsvg::iri::IRI`: 56 bytes, alignment: 8 bytes
print-type-size     variant `Resource`: 56 bytes
print-type-size         field `.0`: 56 bytes
print-type-size     variant `None`: 0 bytes

After (the NodeId is in the heap:
print-type-size type: `librsvg::iri::IRI`: 8 bytes, alignment: 8 bytes
print-type-size     variant `Resource`: 8 bytes
print-type-size         field `.0`: 8 bytes
print-type-size     variant `None`: 0 bytes

In particular, since ComputedValues has the following fields which are
newtypes around IRI:

  clip_path
  marker_end
  marker_mid
  marker_start
  mask

and most of them are unset for most elements, this lets us reduce
CmoputedValues quite a bit:

Before:
print-type-size type: `librsvg::properties::ComputedValues`: 768 bytes, alignment: 8 bytes

After:
print-type-size type: `librsvg::properties::ComputedValues`: 528 bytes, alignment: 8 bytes

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg