Add query clause for outer joins

[jclark]

This support left outer joins.

intermediate-clause :=
   ...
   | outer-join-clause
outer-join-clause :=
   "outer" "join" nullable-binding-pattern
    "in" expression join-condition
nullable-binding-pattern := "var" binding-pattern

The type of every variable bound in binding-pattern will be T?, where T is what it would be normal. When an input frame has no matching entry in the hash table, a frame is emitted in which all the variables occurring in the nullable-binding-pattern are bound to ().

Open points:

• from a Ballerina perspective, join? might be a nicer syntax than outer join.

See also #435

[jclark]

Relates to #445