Implementing AList -> vector (and arrays)

This issue is about implementing an efficient conversion of `'a` alist indexed by `num` into a `'a vector`.

The length of the resulting vector can be left implementation-dependent, but should be long enough to hold the entire alist (in particular, it could just be the max element + 1).

This conversion is very useful when the alist is constructed then frequently looked up. This lookup operation ought to be done in O(1).

One can use this conversion to give `'a sptree` fast lookups as well.

The efficient path to such a conversion is as follows:

1. Set up an array that can hold all the elements
2. Fill the array up
3. **primitively** convert the array into a vector

Steps 1 and 2 can already be done.

The challenging part of this issue is to add the primitive operation to CakeML that converts an array into vector **without copying the array**.