Annotated function types do not work with recursion

[gebner]

let t = unit -> stt unit emp (fun _ -> emp)

fn rec f () : t = _ { f () }

The error message is really bad, because we don't show it in the VS Code extension.

Error: Unexpected error: File "plugin/syntax_extension/PulseSyntaxExtension_Desugar.ml", line 3700, characters 11-16: Pattern matching failed

Error: Please file a bug report, ideally with a minimized version of the source program that triggered the error.

[mtzguido]

I'm at least fixing the crash here: #491