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

test/language/for/truth.wren

@@ -6,27 +6,27 @@ class Iter {
 
 // False and null are false.
 for (n in Iter.new(false)) {
-  IO.print("bad")
+  System.print("bad")
   break
 }
 
 for (n in Iter.new(null)) {
-  IO.print("bad")
+  System.print("bad")
   break
 }
 
 // Everything else is true.
 for (n in Iter.new(true)) {
-  IO.print("true") // expect: true
+  System.print("true") // expect: true
   break
 }
 
 for (n in Iter.new(0)) {
-  IO.print(0) // expect: 0
+  System.print(0) // expect: 0
   break
 }
 
 for (n in Iter.new("")) {
-  IO.print("string") // expect: string
+  System.print("string") // expect: string
   break
 }