Constant folding of compassign and mxassign
Triple component set for cases where the previous value is known (in
addition to the index and assigned value are constant).  Turn A[I]=C
into A=N, where N=A but with N[I]=C.  Same for matrix component assigns.

A practical example of this would be the following code:
    color C = 0;
    C[1] = 0.5;
Now it will turn this into a constant (0, 0.5, 0) instead of assigning
to C and then assigning one component of C separately.
Fix IlmBase 2.1 build breakages
(other minor changes are due to syncing with the analogous file in OIIO)