Allow ranges in list subscript operator.

test/list/subscript_not_num.wren

@@ -1,2 +0,0 @@
-var a = [1, 2, 3]
-a["2"] // expect runtime error: Subscript must be a number.

test/list/subscript_range.wren

@@ -0,0 +1,31 @@
+// Returns lists.
+var list = ["a", "b", "c", "d", "e"]
+IO.print(list[0..0]) // expect: [a]
+IO.print(list[1...1]) // expect: []
+IO.print(list[1..2]) // expect: [b, c]
+IO.print(list[1...2]) // expect: [b]
+IO.print(list[2..4]) // expect: [c, d, e]
+IO.print(list[2...5]) // expect: [c, d, e]
+
+// A backwards range reverses.
+IO.print(list[3..1]) // expect: [d, c, b]
+IO.print(list[3...1]) // expect: [d, c]
+IO.print(list[3...3]) // expect: []
+
+// Negative ranges index from the end.
+IO.print(list[-5..-2]) // expect: [a, b, c, d]
+IO.print(list[-5...-2]) // expect: [a, b, c]
+IO.print(list[-3..-5]) // expect: [c, b, a]
+IO.print(list[-3...-6]) // expect: [c, b, a]
+
+// Half-negative ranges are treated like the negative value is fixed before
+// walking the range.
+IO.print(list[-5..3]) // expect: [a, b, c, d]
+IO.print(list[-3...5]) // expect: [c, d, e]
+IO.print(list[-2..1]) // expect: [d, c, b]
+IO.print(list[-2...0]) // expect: [d, c, b]
+
+IO.print(list[1..-2]) // expect: [b, c, d]
+IO.print(list[2...-1]) // expect: [c, d]
+IO.print(list[4..-5]) // expect: [e, d, c, b, a]
+IO.print(list[3...-6]) // expect: [d, c, b, a]

test/list/subscript_range_from_not_int.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a[1.5..2] // expect runtime error: Range start must be an integer.

test/list/subscript_range_from_too_large.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a[3..2] // expect runtime error: Range start out of bounds.

test/list/subscript_range_from_too_small.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a[-4..2] // expect runtime error: Range start out of bounds.

test/list/subscript_range_to_exclusive_too_large.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a[1...4] // expect runtime error: Range end out of bounds.

test/list/subscript_range_to_exclusive_too_small.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a[0...-5] // expect runtime error: Range end out of bounds.

test/list/subscript_range_to_not_int.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a[1..2.5] // expect runtime error: Range end must be an integer.

test/list/subscript_range_to_too_large.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a[1..3] // expect runtime error: Range end out of bounds.

test/list/subscript_range_to_too_small.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a[0..-4] // expect runtime error: Range end out of bounds.

test/list/subscript_wrong_type.wren

@@ -0,0 +1,2 @@
+var a = [1, 2, 3]
+a["2"] // expect runtime error: Subscript must be a number or a range.