Allow lambda (fn) to have multiple branches, similar to case

[julianhyde]

Standard ML allows a lambda to have multiple branches, similar to case. Thus the following are equivalent

fn [] => 0 | x ::_ => x + 1;
fn list => case list of [] => 0 | x :: _ => x + 1;

but the second currently throws

Encountered " "|" "| "" at line 3, column 12.
Was expecting one of:
    <EOF> 
    "andalso" ...

There are more examples in https://smlhelp.github.io/book/docs/start/syntax/.