Pull in fetch to lt.util.fetch and fix clj->js warning

This pull request is a fix for #1705 (and, implicitly, #1658).

The approach I have taken is to pull in fetch to lt.util.fetch. I realise this approach may not be approved, but I figured that (since the alternative approach is much easier to implement), this pull request would save people time if it happens to be approved. Plus, it makes sense to me.