Report error when writing to const array elements.

It is a small change to functionResolution.cpp. I introduce a new pragma flag that says "when the receiver is constant, the result is constant". Then rely on const checking already in place.

Then there are a few places in the tests where 'const' arrays were updated. I replace 'const' with 'var'.

In one test I used an array literal for initialization of an array, which remained 'const'.