LibJS: Implement string concatenation using ropes

Instead of concatenating string data every time you add two strings
together in JavaScript, we now create a new PrimitiveString that points
to the two concatenated strings instead.

This turns concatenated strings into a tree structure that doesn't have
to be serialized until someone wants the characters in the string.

This *dramatically* reduces the peak memory footprint when running
the SunSpider benchmark (from ~6G to ~1G on my machine). It's also
significantly faster (1.39x) :^)

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