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/literals.wren

@@ -1,10 +1,10 @@
-IO.write(123)     // expect: 123
-IO.write(987654)  // expect: 987654
-IO.write(0)       // expect: 0
-IO.write(-0)      // expect: -0
+IO.print(123)     // expect: 123
+IO.print(987654)  // expect: 987654
+IO.print(0)       // expect: 0
+IO.print(-0)      // expect: -0
 
-IO.write(123.456) // expect: 123.456
-IO.write(-0.001)  // expect: -0.001
+IO.print(123.456) // expect: 123.456
+IO.print(-0.001)  // expect: -0.001
 
 // TODO: Hex? Scientific notation?
 // TODO: Literals at and beyond numeric limits.