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/return/after_else.wren

@@ -1,3 +1,3 @@
-IO.write(fn {
+IO.print(fn {
   if (false) "no" else return "ok"
 }.call) // expect: ok

test/return/after_if.wren

@@ -1,3 +1,3 @@
-IO.write(fn {
+IO.print(fn {
   if (true) return "ok"
 }.call) // expect: ok

test/return/after_while.wren

@@ -1,3 +1,3 @@
-IO.write(fn {
+IO.print(fn {
   while (true) return "ok"
 }.call) // expect: ok

test/return/in_function.wren

@@ -1 +1 @@
-var a = "ok"; IO.write(a) // expect: ok
+var a = "ok"; IO.print(a) // expect: ok

test/return/in_method.wren

@@ -1,8 +1,8 @@
 class Foo {
   method {
     return "ok"
-    IO.write("bad")
+    IO.print("bad")
   }
 }
 
-IO.write((new Foo).method) // expect: ok
+IO.print((new Foo).method) // expect: ok