Clean up text handling a bit:

- Rename IO.write -> IO.print.
- Make IO.write not print a newline.
- Support \u Unicode escapes in strings.

test/list/to_string.wren

@@ -1,17 +1,17 @@
 // Handle empty list.
-IO.write([].toString)             // expect: []
+IO.print([].toString)             // expect: []
 
 // Does not quote strings.
-IO.write([1, "2", true].toString) // expect: [1, 2, true]
+IO.print([1, "2", true].toString) // expect: [1, 2, true]
 
 // Nested lists.
-IO.write([1, [2, [3], 4], 5]) // expect: [1, [2, [3], 4], 5]
+IO.print([1, [2, [3], 4], 5]) // expect: [1, [2, [3], 4], 5]
 
 // Calls toString on elements.
 class Foo {
   toString { return "Foo.toString" }
 }
 
-IO.write([1, new Foo, 2]) // expect: [1, Foo.toString, 2]
+IO.print([1, new Foo, 2]) // expect: [1, Foo.toString, 2]
 
 // TODO: Handle lists that contain themselves.