Add and use efficient string concatenation functions.

This patch modifies most of the string concatenations that show up in
race.swf and slime.swf as causing more than 1 MiB of allocations.

This subsumes #1593.
I wonder if we should completely inline this case. We could keep an `rgbaToCSSStyleConcatArray` (urgh) around and thus would only need to set the actual color values.