Label: make types field a Rc

[Acaccia]

Fixes #603

This was an easy win, simply changing the types field in Label to an Rc<Types> gave instant improvement in performances, as seen on those benchmarks:
criterion.tar.gz (decompress and open report/index.html)

The flamegraph generated from the same benchmark doesn't even show any Label operation anymore:

[aspiwack]

How much time did the benchmark gain overall?

[yannham]

I believe there is no global gain reported in criterion, but randomly clicking through the benchmarks shows that some did not move a lot, and others got anything between roughly a few percents and 30~40%. Several are in the 30-40% zone, which is a nice return for slapping one Rc on top of a struct field. In the end we will still want to fully Rc Types, though, if only for typechecking.