classes.builtin: Add the BUILTIN: keyword which lets builtin classes show up in source files. Probably could use save-location to allow editing, but it seemed to break things.
math.statistics: Fix histogram and collect. Histogram used each-index even when it didn't need to.
help: Mismatched <PRIVATE PRIVATE> pair.
ui: change selected gadgets to a hash-set.
compiler.tree.dead-code.simple: curry live-values.
compiler: more use of hash-sets.