Allow tuples to have rest type

[jclark]

Records can specify a rest type, but tuples cannot. It might be useful to allow this, e.g.

type T (boolean, int...);

This relates to #33, for which this might be needed, depending on the resolution to #27 .

[jclark]

We would also need to add this as a binding pattern and a match pattern. Tuple types would work similarly to closed record types (as delimited by {| |}).

[jclark]

Given the way we have resolved #27, this is what is needed to fix #33.

[jclark]

This also raises the question of what is the difference between a tuple constructor (using round parentheses) and an array constructor (using square brackets). Also relates to #54.