black friday release

Gearing up for Advent of Code maybe. Unclear.

• Add count-distinct and error boundary block trailers
• Add some built-ins for finding common prefixes
• A new integer trailer deletes a specified element in the stack.
• As vaguely planned in the back of my head for a while, assignment variants are gone because they aren't practical. For the hard mutations you want to do, hoards are more flexible and actually reasonable for us to make performant. Now adding trailers to an assignment just pops something and performs the trailers on it.
• Bracket cases now use blocks provided as the condition as predicates rather than naively checking for equality.
• Relatedly, integers, floats, and strings become reluctant blocks that push themselves with a single underscore.
• Add documentation for hoards.
• Improve hoard coercion.