feat: replace IO class with System class and remove separate IO module

- Delete wren_io.c, wren_io.h, and io.wren source files
- Remove multi-arity print overloads and IO.read/IO.time methods
- Add System class with print, printAll, write, writeAll methods to core.wren
- Update all documentation examples and README to use System.print instead of IO.print

doc/site/index.markdown

@@ -6,11 +6,11 @@ Think Smalltalk in a Lua-sized package with a dash of Erlang and wrapped up in
 a familiar, modern [syntax][].
 
     :::dart
-    IO.print("Hello, world!")
+    System.print("Hello, world!")
 
     class Wren {
       flyTo(city) {
-        IO.print("Flying to ", city)
+        System.print("Flying to " + city)
       }
     }
 
@@ -18,7 +18,7 @@ a familiar, modern [syntax][].
       ["small", "clean", "fast"].each {|word| Fiber.yield(word) }
     }
 
-    while (!adjectives.isDone) IO.print(adjectives.call())
+    while (!adjectives.isDone) System.print(adjectives.call())
 
  *  **Wren is small.** The VM implementation is under [4,000 semicolons][src].
     You can skim the whole thing in an afternoon. It's *small*, but not