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/bitwise_not.wren

@@ -1,22 +1,22 @@
-IO.write(~0) // expect: 4294967295
-IO.write(~1) // expect: 4294967294
-IO.write(~23) // expect: 4294967272
+IO.print(~0) // expect: 4294967295
+IO.print(~1) // expect: 4294967294
+IO.print(~23) // expect: 4294967272
 
 // Max u32 value.
-IO.write(~4294967295) // expect: 0
+IO.print(~4294967295) // expect: 0
 
 // Past max u32 value.
-IO.write(~4294967296) // expect: 4294967295
+IO.print(~4294967296) // expect: 4294967295
 
 // Negative numbers.
-IO.write(~-1) // expect: 0
-IO.write(~-123) // expect: 122
-IO.write(~-4294967294) // expect: 4294967293
-IO.write(~-4294967295) // expect: 4294967294
-IO.write(~-4294967296) // expect: 4294967295
+IO.print(~-1) // expect: 0
+IO.print(~-123) // expect: 122
+IO.print(~-4294967294) // expect: 4294967293
+IO.print(~-4294967295) // expect: 4294967294
+IO.print(~-4294967296) // expect: 4294967295
 
 // Floating point values.
-IO.write(~1.23) // expect: 4294967294
-IO.write(~0.00123) // expect: 4294967295
-IO.write(~345.67) // expect: 4294966950
-IO.write(~-12.34) // expect: 11
+IO.print(~1.23) // expect: 4294967294
+IO.print(~0.00123) // expect: 4294967295
+IO.print(~345.67) // expect: 4294966950
+IO.print(~-12.34) // expect: 11