Implicit labels in record expressions

[julianhyde]

As an extension to standard ML, deduce implicit labels in record expressions.

In standard ML, every field in a record expression must have a label 'label = expression'.
With this change, an expression has an implicit label if it is an identifier, an application of a label to an expression, or a field reference (see #9). Thus,

{d, #deptno e, e.salary, three = 1 + 2}

is shorthand for

{d = d, deptno = #deptno e, salary = e.salary, three = 1 + 2}

[julianhyde]

Fixed in b311d20.