Syntax to add a field to the current record

[julianhyde]

We have syntax to add a field, but it adds a new record. For example,

from e in emps,
    pay = e.sal + e.comm

returns elements with fields e and pay. (The e field is a record, containing all 8 fields of emps, and pay is a scalar.)

But we need a syntax to add a field to e, so that it has 9 fields (one of which is pay). Inspired by with (added in #249) which replaces a field with a new value, we could add withnew, like this:

from e in emps
    yield {e withnew pay = e.sal + e.comm}

withnew is not a great name. We should find a better word. But it's good that this can be achieved without adding a new relational operator (type of step).

Elm-like syntax

Elm has extensible records. With Elm's syntax, this example would be

from e in emps
    yield {e | pay = e.sal + e.comm}

[julianhyde]

Oops, this is a duplicate of #61.