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/for.wren

@@ -1,12 +1,12 @@
 // Single-expression body.
-for (i in [1, 2, 3]) IO.write(i)
+for (i in [1, 2, 3]) IO.print(i)
 // expect: 1
 // expect: 2
 // expect: 3
 
 // Block body.
 for (i in [1, 2, 3]) {
-  IO.write(i)
+  IO.print(i)
 }
 // expect: 1
 // expect: 2
@@ -14,7 +14,7 @@ for (i in [1, 2, 3]) {
 
 // Newline after "for".
 for
-(i in [1, 2, 3]) IO.write(i)
+(i in [1, 2, 3]) IO.print(i)
 // expect: 1
 // expect: 2
 // expect: 3