Simple string interpolation.

This allows "%(...)" inside a string literal to interpolate the
stringified result of an expression.

It doesn't support custom interpolators or format strings, but we can
consider extending that later.

test/core/function/call_extra_arguments.wren

@@ -1,7 +1,7 @@
 var f0 = Fn.new { System.print("zero") }
-var f1 = Fn.new {|a| System.print("one " + a) }
-var f2 = Fn.new {|a, b| System.print("two " + a + " " + b) }
-var f3 = Fn.new {|a, b, c| System.print("three " + a + " " + b + " " + c) }
+var f1 = Fn.new {|a| System.print("one %(a)") }
+var f2 = Fn.new {|a, b| System.print("two %(a) %(b)") }
+var f3 = Fn.new {|a, b, c| System.print("three %(a) %(b) %(c)") }
 
 f0.call("a") // expect: zero
 f0.call("a", "b") // expect: zero

test/core/map/to_string.wren

@@ -19,10 +19,10 @@ System.print({1: Foo.new()}) // expect: {1: Foo.toString}
 // will be.
 var s = {1: 2, 3: 4, 5: 6}.toString
 System.print(s == "{1: 2, 3: 4, 5: 6}" ||
-         s == "{1: 2, 5: 6, 3: 4}" ||
-         s == "{3: 4, 1: 2, 5: 6}" ||
-         s == "{3: 4, 5: 6, 1: 2}" ||
-         s == "{5: 6, 1: 2, 3: 4}" ||
-         s == "{5: 6, 3: 4, 1: 2}") // expect: true
+             s == "{1: 2, 5: 6, 3: 4}" ||
+             s == "{3: 4, 1: 2, 5: 6}" ||
+             s == "{3: 4, 5: 6, 1: 2}" ||
+             s == "{5: 6, 1: 2, 3: 4}" ||
+             s == "{5: 6, 3: 4, 1: 2}") // expect: true
 
-// TODO: Handle maps that contain themselves.
+// TODO: Handle maps that contain themselves.