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/string/unicode_escapes.wren

@@ -0,0 +1,18 @@
+// One byte UTF-8 Sequences.
+IO.print("\u0041")     // expect: A
+IO.print("\u007e")     // expect: ~
+
+// Two byte sequences.
+IO.print("\u00b6")     // expect: ¶
+IO.print("\u00de")     // expect: Þ
+
+// Three byte sequences.
+IO.print("\u0950")     // expect: ॐ
+IO.print("\u0b83")     // expect: ஃ
+
+// Capitalized hex.
+IO.print("\u00B6")     // expect: ¶
+IO.print("\u00DE")     // expect: Þ
+
+// TODO: Missing digits in escape.
+// TODO: Syntax for Unicode escapes > 0xffff?