Require annotation on ambiguous bindings

[dburgener]

From a commit message in #205:

Currently this assumes only one valid match. It's possible that we
could have multiple valid matches. For example:

resource read {}
resource write {}

let foo = [ read write ];

In this case, we don't know if foo is supposed to be a list of
permissions or a list of resources until we start using it. As a future
enhancement, we should detect that there are ambiguous possibilities and
prompt the developer to annotate the binding like so:

let foo<perm> = [ read write ];