Our database representation of ASTs does not use assignment patterns, instead encoding the relevant information directly in the associated function/loop/assignment. We convert from an AST with assignment patterns to one without during parsing, so the extractor does not expect any assignment patterns to be present in the AST.
Due to a bug in the parser, this can currently happen for malformed programs. While we should fix that bug once it gets fixed in Acorn, it also makes sense for the extractor to be more robust, so this PR teaches the `ASTExtractor` pass to raise a parse error when it encounters an assignment pattern, and all other passes to simply ignore them.
