Improve syntax for record updates and access #816

I think we should remove the field operator, because, as @lasarojc found out, its type signature is a lie:

>>> val r = {a: 2}

>>> val f = "a"

>>> r.field(f)
static analysis error: error: [QNT000] Record field name must be a string expression but is name: f
Generating record constraints for r,f
Generating constraints for f

r.field(f)
^^^^^^^^^^


2

either f doesn't have type string, or the type signature for field is incorrect.

For the same reason, we should replace with with an update syntax that doesn't pretend to take strings :)

E.g., {r with a : 42}.

We could introduce the ability to pass around labels, perhaps indexed by union types, around. But this would be significantly more involved.