Support rayon-like parallel execution

[nikomatsakis]

Salsa today supports parallelism but in a kind of heavyweight way. It should be possible to easily "map" a query across a long list of inputs using rayon-like parallel processing -- maybe we can even literally use rayon. I haven't put much thought into this. I also want to get rid of the "sendable vs not-sendable" database distinction. Let's just require everything to be sendable and be done with it.