Record types

[andrewbaxter]

Using tuples everywhere relies on positioning to correlate symbols which is risky (mix up the order of two symbols and you can end up with strange results).

Relations kind of support a record syntax, but this isn't supported anywhere else.

I'd like:

• Record rules, like ?{something: a, something_else: b} := .... (Also this would solve the issue that right now there's no way to rename results other than writing a new rule that just forwards the elements with a new name, which is excessive and makes queries more fragile)
• Record matching for non-relations, like := my_rule{a: a, b: b}

I assume the framework for this is already there, since results already have headers etc.

And maybe

• Record values, like y = {a: 4, b: something}
• Record methods, like get(y, "a")