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/number/minus.wren

@@ -1,10 +1,10 @@
 // Infix.
-IO.write(5 - 3)      // expect: 2
-IO.write(3.1 - 0.24) // expect: 2.86
-IO.write(3 - 2 - 1)  // expect: 0
+IO.print(5 - 3)      // expect: 2
+IO.print(3.1 - 0.24) // expect: 2.86
+IO.print(3 - 2 - 1)  // expect: 0
 
 // Unary negation.
 var a = 3
-IO.write(-a) // expect: -3
+IO.print(-a) // expect: -3
 
 // TODO: Unsupported RHS types.