add additional procedural macro links (#1249)
add additional procedural macro links (#1249)

I've been implementing a procedural macro this week and found these
links quite useful to learn the skill.

As an aside, I wonder about the order of links in `links.json`.
I'm curious if the specification declares normative ordering. I've been
informally ordering them using beginning with more practically focused
topics and ending on the more theoretical ones. i.e. a bias toward
practice-based learning materials.
petertseng/exercism-rust,petertseng/exercism-rust,exercism/xrust,exercism/xrust,petertseng/exercism-rust