Implement mb_decode_mimeheader using fast text conversion filters

The new implementation is 2.5x-3x faster.

If an invalid charset name was used, the old implementation would get

Implement mb_decode_mimeheader using fast text conversion filters

The new implementation is 2.5x-3x faster.

If an invalid charset name was used, the old implementation would get
'stuck' trying to parse the charset name and would not interpret any
other MIME encoded words up to the end of the input string. The new
implementation fixes this bug.

If an (invalid) encoded word ends abruptly and a new (valid) encoded
word starts, the old implementation would not decode the valid encoded
word. The new implementation also fixes this.

Otherwise, the behavior of the new implementation has been designed to
closely match that of the old implementation.

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