These two constructs are very simple and similar, and only support 3
different clauses, two of which are already implemented. This patch
adds AST nodes for both constructs, and leaves the device_num clause
unimplemented, but enables the other two.
The 'vector_length' clause is semantically identical to the
'num_workers' clause, in that it takes a mandatory single int-expr. This
is implemented identically to it.