Improve V8-specific performance

@bnoordhuis posted some excellent ideas here:

For example, .toASCII() calls String#split() twice and is heavy on the map-over-substrings idiom. It would probably be faster to:

1. Scan for the split character and only split when found; saves allocating an array in the common case.
2. Iterate rather than map; V8 is not great at lowering/inlining callbacks.
3. Try to avoid concatenating strings too much; cons strings eventually have to be flattened.

There is probably more but that is what stood out from a quick look.

Patches welcome.

cc @trevnorris

Start by locating the .toASCII() implementation and reviewing its String#split(), map-over-substrings, and string-concatenation paths. The issue proposes avoiding unnecessary splits, iterating instead of mapping, and reducing concatenation; done means implementing the relevant improvements while preserving the converter’s existing behavior.