Tracking issue for RFC 1925: Allow an optional vert at the beginning of a match branch

[aturon]

This is a tracking issue for the RFC "Allow an optional vert at the beginning of a match branch " (rust-lang/rfcs#1925).

Steps:

• Implement the RFC -- done in Implement RFC 1925 #44108
  • Test case showing correct behavior
• Adjust documentation (see instructions on forge)
• Stabilization PR (see instructions on forge)

[KiChjang]

I'm interested in looking into this... I suspect most of the code that I need to edit will be under libsyntax, correct?

[KiChjang]

Looks like I was too slow, @mattico has went ahead and created a PR.

[mattico]

I should get in the habit of commenting before working on things... but most things I start I don't finish 😄.