cmp.Diff panics when comparing a struct that has unexported fields and no String
or Equal method, so change some structs to either export all their fields or
have an Equal method.
This fixes#1136.
Also remove support for the "name" field tag from eval.scanOptions - it's not
used anywhere and the use case it was intended for is handled by CamelToDashed.
The `AnyError` placeholder error can cause tests to succeed for errors
other than what was expected. That is, the use of `AnyError` can mask
bugs in a unit test. So replace it with the specific error, or error type,
the test expects to be raised.
This does not remove the anyError structure because it is used in
the TestCase.DoesNotCompile() method. To keep the size of this change
as small as possible I want to defer updating that use to a separate
change. However, remove the public AnyError var so future test writers
don't attempt to use it.
