recent updates

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+ I corrected a problem with lexical scoping blah blah in the main Pollen expander module. The upshot is that certain identifiers that were "accidentally" available before will no longer be. This is consistent with the documented behavior, so I don't consider it a non-backward-compatible change. Yet you may find Pollen source files that now seem broken because they're missing a `require` (for instance, `pollen/pagetree` is one module that was implicitly available before, but is no longer)

+ I fixed a bug in the disk-caching system, but it required changing the format of the cache file. This can be corrected by just running `raco pollen reset` on your project to erase the caches.