jemalloc

Also, `just rewrite it dammit' is not always the right answer. The malloc() in glibc has a lot
of arcane knowledge and nifty speedups for obscure situations in it: replacing it wholesale
would throw all that knowledge away. A malloc() which works slightly better in most
circumstances but drastically worse in some is perhaps not an improvement (not that this is
necessarily true of jemalloc: I haven't checked, but *consistent* behaviour has always been a
major focus of glibc malloc()).