Changed all the instances of Arc to Box

[THE-COB]

• basically ctrl f and replace Arc to Box
• fixed the imports
• tests pass ?????

Addresses https://github.com/drewrip/rascal/issues/6

[drewrip]

For whatever reason I thought the Arc was covering up some bigger issues. I'm glad to hear it was easy (though it probably wasn't that fun).

Thanks for tackling this! Looks good to merge