react: Update only dependents of the just-set cell (#131)
react: Update only dependents of the just-set cell (#131)

* react: Update only dependents of the just-set cell

The initial version of react (see #126) let the reactor store all cells
in a linked list, then blindly updated all cells whenever any cell was
set. This was correct but wasteful.

This commit rearchitects the example solution so that:

* reactors only keep track of the input cells
* all other cells keep track of their dependent compute cells
* only dependents get updated when a cell is set

Space cost of:

* one extra `struct child` allocated per compute2 cell created.
* one extra integer field per cell.

* react: Add NULL checks on all allocations

RealBarrettBrown/xc,RealBarrettBrown/xc,RealBarrettBrown/xc