Unelaborate terms in error messages

[croyzor]

Take the bracketing logic in Brat.Syntax.Core and reimplement it in Brat.Syntax.Flat. When showing error messages to users we should first call unelaborate on the terms to hide the extra information from users and have highlighted terms match actual source code more closely.

We should retain the option of printing elaborated error messages with a new compiler flag, like --verbose-errors

N.B. For nicer printing, we should keep track of whether our :-: and applications were elaborated from |> or ; or f(a)