Allow a `const ArrayHandle` to be reallocated

Previously, the Allocate method of ArrayHandle was not declared as const. Likewise, the methods that depended on Allocate, namely ReleaseResources and PrepareForOutput were also not declared const. The main consequence of this was that if an ArrayHandle were passed as a constant reference argument to a method (e.g. const ArrayHandle<T>& arg), then the array could not be reallocated.

This seems right at first blush. However, we have changed these methods to be const so that you can in fact reallocate the ArrayHandle. This is because the ArrayHandle is in principle a pointer to an array pointer. Such a structure in C will allow you to change the pointer to the array, and so in this context it makes sense for ArrayHandle to support that as well.

Although this distinction will certainly be confusing to users, we think this change is correct for a variety of reasons.

1. This change makes the behavior of ArrayHandle consistent with the behavior of UnknownArrayHandle. The latter needed this behavior to allow ArrayHandles to be passed as output arguments to methods that get automatically converted to UnknownArrayHandle.
2. Before this change, a const ArrayHandle& was still multible is many way. In particular, it was possible to change the data in the array even if the array could not be resized. You could still call things like WritePortal and PrepareForInOut. The fact that you could change it for some things and not others was confusing. The fact that you could call PrepareForInOut but not PrepareForOutput was doubly confusing.
3. Passing a value by constant reference should be the same, from the calling code's perspective, as passing by value. Although the function can change an argument passed by value, that change is not propogated back to the calling code. However, in the case of ArrayHandle, calling by value would allow the array to be reallocated from the calling side whereas a constant reference would prevent that. This change makes the two behaviors consistent.
4. The supposed assurance that the ArrayHandle would not be reallocated was easy to break even accidentally. If the ArrayHandle was assigned to another ArrayHandle (for example as a class' member or wrapped inside of an UnknownArrayHandle), then the array was free to be reallocated.