- This (plus a few more fixes) restore race detection.

- Also get rid of sequential_inames--realize that by a new tag,
  force-sequential.

- Also move loop_nest_map to the scheduler file, because it tweaks
  its results to suit scheduler internals.

- Remove dtype state from reduction.
- Introduce preamble_generators.
- Introduce function_manglers, to replace result_dtype_getters
- Introduce defines.