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/abs.wren

@@ -1,6 +1,6 @@
-IO.write(123.abs)    // expect: 123
-IO.write(-123.abs)   // expect: 123
-IO.write(0.abs)      // expect: 0
-IO.write(-0.abs)     // expect: 0
-IO.write(-0.12.abs)  // expect: 0.12
-IO.write(12.34.abs)  // expect: 12.34
+IO.print(123.abs)    // expect: 123
+IO.print(-123.abs)   // expect: 123
+IO.print(0.abs)      // expect: 0
+IO.print(-0.abs)     // expect: 0
+IO.print(-0.12.abs)  // expect: 0.12
+IO.print(12.34.abs)  // expect: 12.34