Feature request: shorthand syntax for field access

Languages such as Elm have special syntax that simplifies expressions such as `function(x) x.y` into just `.y`; adding support for this shorthand would improve the legibility of Jsonnet code by removing repetitive boilerplate, e.g. this code:

```jsonnet
std.map(function(city) city.name, std.map(function(country) country.capital, countries))
```

could instead be written as

```jsonnet
std.map(.name, std.map(.capital, countries))
```

A function that takes a single argument and just looks at a field on it is very common, e.g.

```jsonnet
std.filter(.isPublic, files)
std.flatMap(.members, groups)
```