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/while/closure_in_body.wren

@@ -3,7 +3,7 @@ var list = []
 var i = 1
 while (i < 4) {
   var j = i + 1
-  list.add(fn IO.write(j))
+  list.add(fn IO.print(j))
   i = i + 1
 }
 

test/while/syntax.wren

@@ -1,6 +1,6 @@
 // Single-expression body.
 var c = 0
-while (c < 3) IO.write(c = c + 1)
+while (c < 3) IO.print(c = c + 1)
 // expect: 1
 // expect: 2
 // expect: 3
@@ -8,7 +8,7 @@ while (c < 3) IO.write(c = c + 1)
 // Block body.
 var a = 0
 while (a < 3) {
-  IO.write(a)
+  IO.print(a)
   a = a + 1
 }
 // expect: 0
@@ -18,7 +18,7 @@ while (a < 3) {
 // Newline after "while".
 var d = 0
 while
-(d < 3) IO.write(d = d + 1)
+(d < 3) IO.print(d = d + 1)
 // expect: 1
 // expect: 2
 // expect: 3