Add ops for working with arrays and collections

E.g., `a + b` for two arrays `a` and `b`:

``` java
String[] both = Stream.concat(Arrays.stream(a), Arrays.stream(b)).toArray(String[]::new);
```

or

``` java
String[] both = Stream.of(a, b).flatMap(Stream::of).toArray(String[]::new);
```

And similar stuff for collections. Will be really nice to have this stuff in `eval`.

The issue names no files, tests, or entry points. Start by locating the eval implementation and determine how array and collection operations are registered; define the supported behaviors and verify them with focused tests.

Written by the indexing model from the issue text.