- Rework conversion descriptors again. New version gives me nice inline conversions, which are used both for immediate conversion adjustment and for tiles. Autogenerated conversion code for inline arguments.

- Rework the cost model to make sense. Inline conversions inside tiles have cost, at codegen are free, and register/outline conversions have a different cost. Also, give impls a base cost --- since different impls of the same operation may cost differently; a cost of a tile is hence the base cost of the impl plus the cost of any conversions inlined into tile. This is stashed inside the opcode descriptor, of course.

- Start sketching out the actual code generator, and stop, realizing I still need register management infrastructure. Not sure what the mostrobust way of doing it yet.