Vectors

Add compound data type "Vector" to SMPL

**Description: **
> A vector is an array that it is not constrained to hold only one type of data (i.e. a heterogeneous array). Vector initialisation is quite flexible. A vector may be initialised by specifying a collection of disjoint subvectors, or the individual elements, or a combination of the two. A subvector is specified by two expressions: the first (after it has been evaluated) gives the size of the subvector, the second (after it has been evaluated) must be a procedure that when given an index less than the size of the subvector, returns the value to be stored at that position in the subvector.

**Ops: **
- `[: [e1,.....,en] :] // Return a newly allocated vector initialised with the given specifications.
> Each specification is either an expression or of the form expr size : expr init.

The issue provides the Vector semantics and initialization syntax but names no files, tests, or entry points. Start by surveying the SMPL interpreter to find where compound types and expression syntax are implemented; the work is done when heterogeneous vectors support the specified element and subvector forms.

Written by the indexing model from the issue text.