Per-component reducers, chosen and bound once when the simulation is built.

 fastReduce works out afresh, for every component of every clock step, things that were
 settled when the simulation was built: which component type this is, whether it is on the
 uint32 or the bigint path, which of its inputs are bigints, what its bus masks are, and
 which arrays its ports live in. A reducer built here has all of that already: it closes
 over the step arrays themselves and over the constants, and its body is only the work that
 actually differs from step to step.

 reducerFor returns None for a component type it does not handle, and the caller falls back
 to fastReduce. So this file can be filled in a type at a time, and the fallback is always
 the definition of what a reducer must do.

 Two rules the closures depend on, both guaranteed by the caller (see installReducers):

  - The step arrays captured here must be the ones the simulation actually uses. Reducers
    are therefore installed after every re-linking pass, including the one
    addWavesToFastSimulation does for custom components, and only for components that are
    reduced. Capturing an array that is later re-pointed would silently simulate the wrong
    signal.
  - Values in the step arrays are already within their bus width (the masking invariant), so
    a reducer masks its result exactly when its own operation can overflow, and reads
    without masking. On the uint32 path a width of exactly 32 needs care: 1u <<< 32 is 1u.