Implement \cases (Proposal 302)wip/T20768

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 559e41505e81d93939e9afa6aa9793b0a428924
+Subproject 807534e45e934def00d5a631c715ddeacfd691c