Require that array literals produce "rectangular" arrays, i.e. all the

subarrays of a given dimension have the same number of elements/subarrays.

Also repair a longstanding undocumented (as far as I can see) ability to
explicitly set array bounds in the array literal syntax. It now can
deal properly with negative array indicies. Modify array_out so that
arrays with non-standard lower bounds (i.e. not 1) are output with
the expicit dimension syntax. This fixes a longstanding issue whereby
arrays with non-default lower bounds had them changed to default
after a dump/reload cycle.

Modify regression tests and docs to suit, and add some minimal
documentation regarding the explicit dimension syntax.