feat: add MapEntry class and make Map a Sequence for direct iteration

Add MapEntry class to expose key-value pairs during map iteration, and make Map extend Sequence so maps can be directly iterated with `for` loops. Rename the internal `iterate_` primitive to `iterate` and update error messages for invalid map iterators.

test/core/map/is_empty.wren

@@ -0,0 +1,2 @@
+System.print({}.isEmpty) // expect: true
+System.print({1: 1}.isEmpty) // expect: false

test/core/map/iterate.wren

@@ -0,0 +1,45 @@
+var a = {"one": 1, "two": 2, "three": 3, "four": 4}
+
+// The precise numeric values aren't defined since they are indexes into the
+// entry table and the hashing process isn't specified. So we just validate
+// what we can assume about them.
+
+System.print(a.iterate(null) is Num) // expect: true
+System.print(a.iterate(null) >= 0) // expect: true
+
+System.print(a.iterate(0) is Num) // expect: true
+System.print(a.iterate(0) > 0) // expect: true
+System.print(a.iterate(1) is Num) // expect: true
+System.print(a.iterate(1) > 0) // expect: true
+System.print(a.iterate(2) is Num) // expect: true
+System.print(a.iterate(2) > 0) // expect: true
+System.print(a.iterate(3) is Num) // expect: true
+System.print(a.iterate(3) > 0) // expect: true
+
+var previous = -1
+var iterator = a.iterate(null)
+while (iterator) {
+  System.print(iterator > previous)
+  System.print(iterator is Num)
+  previous = iterator
+  iterator = a.iterate(iterator)
+}
+// First entry:
+// expect: true
+// expect: true
+// Second entry:
+// expect: true
+// expect: true
+// Third entry:
+// expect: true
+// expect: true
+// Fourth entry:
+// expect: true
+// expect: true
+
+// Out of bounds.
+System.print(a.iterate(16)) // expect: false
+System.print(a.iterate(-1)) // expect: false
+
+// Nothing to iterate in an empty map.
+System.print({}.iterate(null)) // expect: false

test/core/map/iterate_iterator_not_int.wren

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+var a = {1: 2, 3: 4}
+a.iterate(1.5) // expect runtime error: Iterator must be an integer.

test/core/map/iterate_iterator_not_num.wren

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+var a = {1: 2, 3: 4}
+a.iterate("2") // expect runtime error: Iterator must be a number.

test/core/map/iterator_value.wren

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+var a = {1: "one"}
+
+// The actual iterator values are implementation specific, so ask the map.
+var iterator = a.iterate(null)
+var value = a.iteratorValue(iterator)
+
+System.print(value is MapEntry) // expect: true
+System.print(value.key)         // expect: 1
+System.print(value.value)       // expect: one
+
+// The entry does not track the underlying map.
+a[1] = "updated"
+System.print(value.value)       // expect: one

test/core/map/iterator_value_iterator_not_int.wren

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+var a = {1: "one"}
+a.iteratorValue(1.5) // expect runtime error: Iterator must be an integer.

test/core/map/iterator_value_iterator_not_num.wren

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+var a = {1: "one"}
+a.iteratorValue("2") // expect runtime error: Iterator must be a number.

test/core/map/iterator_value_iterator_too_large.wren

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+var a = {1: "one"}
+
+// The maximum value is based on the map's capacity, not its count, so use a
+// sufficiently large enough value for the test to make not affected by growth
+// strategy.
+a.iteratorValue(9999) // expect runtime error: Iterator out of bounds.

test/core/map/iterator_value_iterator_too_small.wren

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+var a = {1: "one"}
+
+// The maximum value is based on the map's capacity, not its count, so use a
+// sufficiently large enough value for the test to make not affected by growth
+// strategy.
+a.iteratorValue(-9999) // expect runtime error: Iterator out of bounds.