Add a generic `contains` implementation on the `Sequence` base class, removing the duplicate method from `List`. This allows all sequence types (including Range) to use the method without per-class redefinition. Include new test files for `List.contains` and `Range.contains` covering ordered, backwards, and exclusive ranges.
The benchmark directory is relocated from the project root into the test directory, and all scripts, documentation links, and gitignore paths are updated accordingly. The test runner is also refactored to explicitly walk subdirectories (core, io, language, limit) instead of using a single test root, and a new test/README.md is added to document the test suite structure. Additionally, the constructor test for the Foo class is updated to add a zero-arity constructor and adjust expected output strings.
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.
