Implement a new `count(f)` method on the Sequence class that accepts a predicate function and returns the number of elements for which the predicate evaluates to true. The implementation iterates over the sequence, calling `f.call(element)` on each element and incrementing a counter when the result is truthy.
Also update the documentation for both Sequence and List classes: add full documentation for the new `count(predicate)` method on Sequence, and change the terminology in List docs from "items" to "elements" for consistency. Add three test files covering the basic predicate behavior, non-boolean return values from the predicate, and the error case when a non-function argument is passed.
