new join syntax

[mccanne]

The join operator should be changed so that it takes an as clause to assign var names to the left and right inputs (defaulting without the as clause to "left" and "right"), then the expression (that used to be called the join assignments) is just an expression that is evaluated that can reference left and right. Now that we have record spreads, it would be nice this way to just say...

join on x=y {...left, x:right.x}

If the expression is omitted, the default behavior should be to join the entire value from left and right, e.g.,

{...left, ...right}