FlatAffineConstraints API update - additional methods

- add methods addConstantLowerBound, addConstantUpperBound, setIdToConstant,
  addDimsForMap
- update coefficient storage to use numReservedCols * rows instead of numCols *
  rows (makes the code simpler/natural; reduces movement of data when new
  columns are added, eliminates movement of data when columns are added to the
  end).

(addDimsForMap is tested in the child CL on memref bound checking: cl/219000460)

PiperOrigin-RevId: 219358376