Implement codecs.ignore_errors

[BCSharp]

It works, but I feel uneasy about using a type which name starts with an underscore, although the type itself is public. Is it the right way of doing? If not, here is an alternative implementation of ignore_errors, which I can push in the next commit:

private static object IgnoreErrors(CodeContext context, object unicodeError) {
    switch (unicodeError) {
        case PythonExceptions.BaseException _:
            var ptype = DynamicHelpers.GetPythonType(unicodeError);
            if (ptype.Name == "UnicodeDecodeError" || ptype.Name == "UnicodeEncodeError") {
                if (ptype.TryGetBoundAttr(context, unicodeError, "end", out object ret) && ret is int end) {
                    return PythonTuple.MakeTuple(string.Empty, end);
                }
            }
            goto default;
        case DecoderFallbackException dfe: 
            return PythonTuple.MakeTuple(string.Empty, dfe.Index + dfe.BytesUnknown.Length);
        case EncoderFallbackException efe: 
            return PythonTuple.MakeTuple(string.Empty, efe.Index + (efe.CharUnknownHigh != '\0' ? 2 : 1));
        default: throw PythonOps.TypeError("codec must pass exception instance");
    }
}

[slozier]

Underscore types seem to mostly be used within the exception classes, however I don't have any better suggestions so this works for me. Have you looked at the tests for these in test_codecscallback?

[BCSharp]

I haven't noticed test_codeccallbacks.py until now. This is great. The tests I am writing are just enough to guide the implementation; they are by no means exhaustive. test_codeccallbacks.py is invaluable. Right now, almost all tests from that file fail, but looking at the failures I see they are expected. So there is still some way to go.