Use constants for method constraints in BoxDrawing

[jjatria]

Rakudo does not seem to allow constraints that use hashes in role methods (see https://github.com/rakudo/rakudo/issues/3172), but the issue disappears if the hashes are declared as constants rather.

At least until the issue is resolved upstream, this allows classes to consume the BoxDrawing role without needing to disable precompilation.

Fixes #74.

[jjatria]

Hm, actually, let me add some tests for this.

[jjatria]

I've added some extremely non-DRY tests for this. Maybe tomorrow I can be more clever about how to avoid repeating myself repeating myself.

[japhb]

To make the tests simpler and shorter, why not .join the grids, so you can just compare each against a single nine-character string? Then you could even make a little helper:

sub border-check($expected, $style) {
    given A.new(x => 0, y => 0, h => 3, w => 3) {
        .draw-box(0, 0, 2, 2, ($style if $style));
        is .grid.grid.join, $expected, $style || $default;
    }
}

... and similarly for border-check-with-color or so.

[jjatria]

I've updated the tests, so they're more DRY now

[japhb]

Merged, thanks!