Raster-compiled log densities as foerster blocks (doc/contract.md).

A raster block is a `deftm` log density over typed arguments. The block
says how θ and the site's inputs become those arguments, and which argument
slots hold θ:

  (raster-block {:block/id :gauss
                 :block/latents [{:name :mu :shape [2] :support :real}]
                 :block/target :complete-conditional}
                #'gauss-lp
                {:args  (fn [^doubles theta {:keys [ys n s0 s]}]
                          [(aget theta 0) (aget theta 1) ys n s0 s])
                 :theta [0 1]})

`:theta` lists, in θ order, the argument slots θ occupies: a scalar slot
contributes one coordinate, an array slot as many as its gradient has.
The block's `:log-density` calls the compiled function; its `:value+grad`
calls raster's reverse mode (`raster.ad.reverse/value+grad`) with respect
to the θ slots only, built once per block: the other arguments (the data)
stay constant. Raster owns compilation and caching; this namespace only
binds arguments.