(The old syntax is still supported as well, for now.)
It is now possible to leave out the parens around if, while, and
do/while conditions, and around alt expressions. Cases in an alt block
can now leave off the case keyword and parens around the pattern.
After the next snapshot, we can start migrating our code to use the
new alt syntax, probably with a pretty-printer pass. The paren-free
syntax will remain optional (you may always parenthesize expressions),
but the old case syntax will no longer be supported in the future.
