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/break/in_for_loop.wren

@@ -1,7 +1,7 @@
 for (i in [1, 2, 3, 4, 5]) {
-  IO.write(i)
+  IO.print(i)
   if (i > 2) break
-  IO.write(i)
+  IO.print(i)
 }
 // expect: 1
 // expect: 1

test/break/in_while_loop.wren

@@ -1,9 +1,9 @@
 var i = 0
 while (true) {
   i = i + 1
-  IO.write(i)
+  IO.print(i)
   if (i > 2) break
-  IO.write(i)
+  IO.print(i)
 }
 // expect: 1
 // expect: 1

test/break/nested_loop.wren

@@ -1,11 +1,11 @@
 var i = 0
 while (true) {
-  IO.write("outer " + i.toString)
+  IO.print("outer " + i.toString)
   if (i > 1) break
 
   var j = 0
   while (true) {
-    IO.write("inner " + j.toString)
+    IO.print("inner " + j.toString)
     if (j > 1) break
 
     j = j + 1