Check that functions in an `impl` are declared properly. For now, simply
reuse the redeclaration checking logic. Longer-term, we need to check
that the signatures match in a more approximate way, after substituting
the `Self` type into the signature in the `interface`.
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Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
This detects more cases of incorrect matches between declaration and
definition. It also factors the logic out to a separate file for easier
sharing, particularly when it comes to merging imports (I'm not sure if
this exact API will be reshared, but the core logic should apply).
Richard SmithandJon Ross-Perkins