support access to array members

[StephanWald]

DIM X[19,3]
X[1,3]=12

dim Y$[10]
y$[1](2,1)="TEST"

[dhuebner]

• [x] X[1,3]=12 allow array elements be the left side of assignment
• [x] y$[1](2,1)="TEST" allow substing on array elements

[dhuebner]

The example mentioned above are working now.