Clean up/add warnings to use of triggers

[robin-aws]

Follow-up from #7 (comment): we agree that while it is reasonable for a Dafny standard library to make limited use of triggers, we want to ensure that Dafny programmers are strongly encouraged to avoid triggers in their own code.

[parno]

This is currently waiting on PR #25, which in turn is waiting on review from @RustanLeino. Once that lands, @sydgibs has a follow on set of patches to add triggers to the rest of the library.