consider exporting a compare-and-swap or conditional-store

[rme]

I've seen some interest in using atomic operations in CL implementations. See, for example,  http://kvardek-du.kerno.org/2012/06/augmenting-bordeaux-threads-with-atomic.html.

We've had ccl::conditional-store for quite some time, but it's not really fully baked. On IRC, sykopomp reported that it choked on typed defstruct slots because in that case we wrap ccl::struct-ref as (the struct-type (ccl::struct-ref ...)).