implement store subscript

[tdp2110]

Like python:

>>> import dis
>>> def store(x, ix, v):
...     x[ix] = v
...
>>> dis.dis(store)
  2           0 LOAD_FAST                2 (v)
              2 LOAD_FAST                0 (x)
              4 LOAD_FAST                1 (ix)
              6 STORE_SUBSCR
              8 LOAD_CONST               0 (None)
             10 RETURN_VALUE

[tdp2110]

done in bytecode in 3ee9b63 and in treewalk in eff787c