Performance issues during supplementary view layout

[jwilling]

During profiling, I noticed that there were several issues in the layout of supplementary views that is only noticeable if there are a large number of supplementary views.

1. The reuse identifier for supplementary views is stored as a combination of the kind and the reuse identifier, separated by the / character. This is nice, except for the fact that when we want to retrieve the kind or the reuse identifier it requires the call to componentsSeparatedByString:, which is unfortunately a bit slow. It seems like this should be unnecessary. This might be good to look into.
2. Unlike the cells, the calls to determine if a section is inside a rect are not forwarded to the layout. Instead, it's done by a linear search through the sections. This is quite inefficient when there are a large number of sections, and the layout could much more effectively handle this.