Implement \cases (Proposal 302)

This commit implements proposal 302: \cases - Multi-way lambda
expressions.

This adds a new expression heralded by \cases, which works exactly like
\case, but can match multiple apats instead of a single pat.

Updates submodule haddock to support the ITlcases token.

Closes #20768

1 files changed, 0 insertions, 0 deletions

diff --git a/utils/haddock b/utils/haddock
-Subproject 58237d76c96325f25627bfd7cdad5b93364d29a
+Subproject fb0e9bac0a5297f995b151f25aa1ce3e622e12e