Initial MVP implementation for [RFC 110](NixOS/rfcs#110)

This reverts commit 1a22abd.

In preparation for revised implementation

Before:
```
[ inherit (attrs) a b c; ] := builtins.attrValues { inherit (attrs) a b c; }
```

After:
```
[ inherit (attrs) a b c; ] := [ attrs.a attrs.b attrs.c ];
```

The previous desugaring has some potentially nice properties such as
non-significant ordering and no-duplicate enforcement, but was
ultimately deemed unintuitive and too surprising in practical use.

The new syntax is

```
attrs.[ a b c ]
```

as sugar for

```
[ attrs.a attrs.b attrs.c ]
```