Add attrpath notation equivalent to `elemAt` about nix HOT 1 OPEN

I personally think the cost benefit works out slightly in favor of having this feature.
It's rare to need elemAt, and most functions should not use it, but it's useful in the repl.

We don't get to change usable syntax (perhaps until NixOS/rfcs#137), so we don't have a lot of choice.

attrpath.0

Unfortunately, this is equivalent to attrpath .0, a function application with argument 0.0.

attrpath[0]

Same, but with a list.

attrpath@0

Currently an @ always creates a binding. I'd like to reserve it for that purpose.

attrpath.${0}

This would make type inference worse (e.g. in https://github.com/oxalica/nil), as the lhs can be a list or attribute set, and the rhs can be an int or string. Also the property that the types need to match up may be hard to express in a type system.

How about...

• attrpath.$[0]

It being distinct makes type inference easy, and helps readers.

[ reminds of lists, and $[ is still available as far as the grammar's concerned.

from nix.