[C++] Add function for retrieving a scalar from an array slot

It'd be useful to construct scalar values given an array and an index.

/* static */ std::shared_ptr<Scalar> Scalar::FromArray(const Array&, int64_t);

Since this is much less efficient than unboxing the entire array and accessing its buffers directly, it should not be used in hot loops.

Krisztian Szucs Francois Saint-Jacques