malloc cache/trace preview

[DJ Delorie]

I just pushed to branch dj/malloc two features I've been working on,
for preview and criticism:

First, a per-thread cache that speeds up small (<1024) allocations by
up to 5-10x (average 2x).

Second, the start of a low-overhead workload tracing feature that can
be (eventually) enabled for an existing program to capture its malloc
et al calls and later replay them for benchmarking.

DJ