Benchmark script / more consistent string concatenation syntax

[muellermichel]

@softvar: Not sure if you like this syntax better, but it's more consistent with the rest of the code and a bit shorter - see whether you want to merge it or not. In addition I've created a little benchmark script.

Result of benchmark on my recent-ish iMac 27'' with the proposed script: ~0.54 seconds for a 6000 layer nested table. At 7000 layers it runs into a segfault (too many intermediate objects?). Anyways, performance is probably not a big issue with this library, i.e. python itself is a bigger bottleneck.