postal-mime: Quadratic CPU consumption when decoding a header of many joinable RFC 2047 base64 encoded words
### Impact Decoding a header that holds a long run of joinable RFC 2047 base64 encoded words, such as `=?utf-8?B?QUJD?=` repeated tens of thousands of times in a `Subject`, `From`, `To`, a filename or a description, took time quadratic in the number of words. The join check ran a regex over the text joined so far for every word, which flattens the accumulated string each time. A single well formed message of about 1.6 MB blocked the event loop for one to twenty seconds depending on the host, and inline `message/rfc822` parts multiplied the cost, since each nested message carried its own header budget. Any application that parses untrusted m
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