feat: replace eager map/where with lazy MapSequence and WhereSequence classes

Refactor Sequence.map and Sequence.where to return lazy MapSequence and WhereSequence wrappers instead of eagerly building a List. Add new MapSequence and WhereSequence classes that delegate iteration and apply the transformation or predicate on-the-fly. Update documentation to reflect lazy semantics and the need to call .list to materialize results. Adjust all existing tests to call .list on map/where results. Add new tests in test/core/sequence/ verifying lazy behavior with infinite Fibonacci iterators.

test/core/sequence/map.wren

@@ -0,0 +1,46 @@
+// Infinite iterator demonstrating that Sequence.map is not eager
+class FibIterator {
+  new {
+    _current = 0
+    _next = 1
+  }
+
+  iterate {
+    var sum = _current + _next
+    _current = _next
+    _next = sum
+  }
+
+  value { _current }
+}
+
+class Fib is Sequence {
+  iterate(iterator) {
+    if (iterator == null) return new FibIterator
+    iterator.iterate
+    return iterator
+  }
+
+  iteratorValue(iterator) { iterator.value }
+}
+
+var squareFib = (new Fib).map {|fib| fib * fib }
+var iterator = null
+
+IO.print(squareFib is Sequence) // expect: true
+IO.print(squareFib) // expect: instance of MapSequence
+
+iterator = squareFib.iterate(iterator)
+IO.print(squareFib.iteratorValue(iterator)) // expect: 0
+
+iterator = squareFib.iterate(iterator)
+IO.print(squareFib.iteratorValue(iterator)) // expect: 1
+
+iterator = squareFib.iterate(iterator)
+IO.print(squareFib.iteratorValue(iterator)) // expect: 1
+
+iterator = squareFib.iterate(iterator)
+IO.print(squareFib.iteratorValue(iterator)) // expect: 4
+
+iterator = squareFib.iterate(iterator)
+IO.print(squareFib.iteratorValue(iterator)) // expect: 9

test/core/sequence/where.wren

@@ -0,0 +1,40 @@
+// Infinite iterator demonstrating that Sequence.where is not eager
+class FibIterator {
+  new {
+    _current = 0
+    _next = 1
+  }
+
+  iterate {
+    var sum = _current + _next
+    _current = _next
+    _next = sum
+  }
+
+  value { _current }
+}
+
+class Fib is Sequence {
+  iterate(iterator) {
+    if (iterator == null) return new FibIterator
+    iterator.iterate
+    return iterator
+  }
+
+  iteratorValue(iterator) { iterator.value }
+}
+
+var largeFibs = (new Fib).where {|fib| fib > 100 }
+var iterator = null
+
+IO.print(largeFibs is Sequence) // expect: true
+IO.print(largeFibs) // expect: instance of WhereSequence
+
+iterator = largeFibs.iterate(iterator)
+IO.print(largeFibs.iteratorValue(iterator)) // expect: 144
+
+iterator = largeFibs.iterate(iterator)
+IO.print(largeFibs.iteratorValue(iterator)) // expect: 233
+
+iterator = largeFibs.iterate(iterator)
+IO.print(largeFibs.iteratorValue(iterator)) // expect: 377