Tile group arrays

[sampsyo]

In the config section, it should be possible to declare—in addition to individual groups—arrays of tile groups. A single group looks like this:

arrange t as {
   group foo at (...);
}

An array of tile groups might do this:

arrange t as {
   group foo[i] at (...);
}

In this setting, the at range for the tile group array should probably be able to refer to i, i.e., the index in the tile group array.

We should also probably support multi-dimensional (at least 2D) arrays.

[epeguero]

No code-gen for this yet, but the syntax parses and pattern-matching works!