Use deferred execution for Sequence.map and Sequence.where

The methods Sequence.map and Sequence.where are now implemented using
deferred execution. They return an instance of a new Sequence-derived
class that performs the operation while iterating. This has three main
advantages:

* It can be computationally cheaper when not the whole sequence is
  iterated.

* It consumes less memory since it does not store the result in a newly
  allocated list.

* They can work on infinite sequences.

Some disadvantages are:

* Iterating the returned iterator will be slightly slower due to
  the added indirection.

* You should be aware that modifications made to the original sequence
  will affect the returned sequence.

* If you need the result in a list, you now need to call Sequence.list
  on the result.

test/core/list/map.wren

@@ -1,3 +1,3 @@
 var a = [1, 2, 3]
-var b = a.map {|x| x + 1 }
+var b = a.map {|x| x + 1 }.list
 IO.print(b) // expect: [2, 3, 4]

test/core/list/where.wren

@@ -1,6 +1,6 @@
 var a = [1, 2, 3]
-var b = a.where {|x| x > 1 }
+var b = a.where {|x| x > 1 }.list
 IO.print(b) // expect: [2, 3]
 
-var c = a.where {|x| x > 10 }
+var c = a.where {|x| x > 10 }.list
 IO.print(c) // expect: []

test/core/range/map.wren

@@ -1,3 +1,3 @@
 var a = 1..3
-var b = a.map {|x| x + 1 }
+var b = a.map {|x| x + 1 }.list
 IO.print(b) // expect: [2, 3, 4]

test/core/range/where.wren

@@ -1,6 +1,6 @@
 var a = 1..3
-var b = a.where {|x| x > 1 }
+var b = a.where {|x| x > 1 }.list
 IO.print(b) // expect: [2, 3]
 
-var c = a.where {|x| x > 10 }
+var c = a.where {|x| x > 10 }.list
 IO.print(c) // expect: []

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