In implementation of Array, replace Cell<RawArray> with RefCell

[pliniker]

Originally, Array didn't have runtime-borrow capability and I made the (hasty) decision to base the Array struct on using Cell<RawArray<T>>.

It is clear that it makes far more sense to use RefCell now, both from a semantic and performance basis. The explicit borrowflag can also be removed.

Source: interpreter/src/array.rs:42
Book: booksrc/chapter-interp-arrays.md