Clean up type validation on lists #205
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If we get a list in a binding like:
```
let foo = [ bar baz qux ];
```
We don't know what types to expect when validating. That could cause
the validation to assume the wrong type, if eg bar is both a domain and
a permission. So we try our best, and if the list is inconsistent, we
see what types we got and try revalidating the full list against each of
those types. If we get a match, we use that, otherwise we error.
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 ];
```
But that is future work, not part of this PR
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#199 caused some of our type matching on lists to start to fail. First clean up the error message when this happens, which was previously a TODO, then try harder to infer the right types. As mentioned in the second commit message, this has us guess (possibly wrongly) on ambiguity. For a post-0.1 future enhancement, we should detect ambiguity and require the developer to annotate.