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/range.wren

@@ -1,12 +1,12 @@
 var inclusive = 2..5
-IO.write(inclusive is Range) // expect: true
-IO.write(inclusive.min) // expect: 2
-IO.write(inclusive.max) // expect: 5
+IO.print(inclusive is Range) // expect: true
+IO.print(inclusive.min) // expect: 2
+IO.print(inclusive.max) // expect: 5
 
 var exclusive = 2...5
-IO.write(exclusive is Range) // expect: true
-IO.write(exclusive.min) // expect: 2
-IO.write(exclusive.max) // expect: 4
+IO.print(exclusive is Range) // expect: true
+IO.print(exclusive.min) // expect: 2
+IO.print(exclusive.max) // expect: 4
 
 // TODO: Non-number RHS.
 // TODO: Non-integer RHS.