Email::MIME eats memory, wastes time parsing out all the
headers, and some problems can't be fixed without breaking
compatibility for other projects which depend on it.
Informal benchmarks show a ~2x improvement in general
stats gathering scripts and ~10% improvement in HTML
view rendering.
We also don't need the ability to create MIME messages, just
parse them and maybe drop an attachment.
While this isn't the zero-copy or streaming MIME parser of my
dreams; it's still an improvement in that it doesn't keep a
scalar copy of the raw body around along with subparts. It also
doesn't parse subparts up front, so it can also replace our uses
of Email::Simple.