Fix slice

[ofavre]

slice was broken with an offset or length above 1000. Shopify documents that slice supports arrays in addition to strings; I added support for working on slices.

Checklist

• [x] I have read the contribution guidelines.
• [x] make test passes.
• [x] make lint passes. (at least on modified files)
• [x] New and changed code is covered by tests.
• [ ] Performance improvements include benchmarks.
• [x] Changes match the documented (not just the implemented) behavior of Shopify.

[danog]

Hi! Mind rebasing this? :)