"IO" -> "System".

Get rid of the separate opt-in IO class and replace it with a core
System class.

- Remove wren_io.c, wren_io.h, and io.wren.
- Remove the flags that disable it.
- Remove the overloads for print() with different arity. (It was an
  experiment, but I don't think it's that useful.)
- Remove IO.read(). That will reappear using libuv in the CLI at some
  point.
- Remove IO.time. Doesn't seem to have been used.
- Update all of the tests, docs, etc.

I'm sorry for all the breakage this causes, but I think "System" is a
better name for this class (it makes it natural to add things like
"System.gc()") and frees up "IO" for referring to the CLI's IO module.

test/language/constructor/cannot_be_infix.wren

@@ -1,5 +1,5 @@
 class Foo {
   this +(value) { // expect error
-    IO.print("ok")
+    System.print("ok")
   }
 }

test/language/constructor/cannot_be_minus.wren

@@ -1,5 +1,5 @@
 class Foo {
   this -(value) { // expect error
-    IO.print("ok")
+    System.print("ok")
   }
 }

test/language/constructor/cannot_be_setter.wren

@@ -1,5 +1,5 @@
 class Foo {
   this name=(value) { // expect error
-    IO.print("ok")
+    System.print("ok")
   }
 }

test/language/constructor/cannot_be_subscript.wren

@@ -1,5 +1,5 @@
 class Foo {
   this [value] { // expect error
-    IO.print("ok")
+    System.print("ok")
   }
 }

test/language/constructor/cannot_be_unary.wren

@@ -1,5 +1,5 @@
 class Foo {
   this ! { // expect error
-    IO.print("ok")
+    System.print("ok")
   }
 }

test/language/constructor/cannot_call_initializer.wren

@@ -1,6 +1,6 @@
 class Foo {
   construct new() {
-    IO.print("ok")
+    System.print("ok")
   }
 }
 

test/language/constructor/named.wren

@@ -5,10 +5,10 @@ class Foo {
   toString { _field }
 }
 
-IO.print(Foo.named()) // expect: named
-IO.print(Foo.other()) // expect: other
+System.print(Foo.named()) // expect: named
+System.print(Foo.other()) // expect: other
 
 // Returns the new instance.
 var foo = Foo.named()
-IO.print(foo is Foo) // expect: true
-IO.print(foo.toString) // expect: named
+System.print(foo is Foo) // expect: true
+System.print(foo.toString) // expect: named

test/language/constructor/no_parameter_list.wren

@@ -1,5 +1,5 @@
 class Foo {
   this new { // expect error
-    IO.print("ok")
+    System.print("ok")
   }
 }

test/language/constructor/superclass.wren

@@ -1,6 +1,6 @@
 class A {
   construct new(arg) {
-    IO.print("new A ", arg)
+    System.print("new A " + arg)
     _field = arg
   }
 
@@ -10,7 +10,7 @@ class A {
 class B is A {
   construct new(arg1, arg2) {
     super(arg2)
-    IO.print("new B ", arg1)
+    System.print("new B " + arg1)
     _field = arg1
   }
 
@@ -20,7 +20,7 @@ class B is A {
 class C is B {
   construct new() {
     super("one", "two")
-    IO.print("new C")
+    System.print("new C")
     _field = "c"
   }
 
@@ -31,10 +31,10 @@ var c = C.new()
 // expect: new A two
 // expect: new B one
 // expect: new C
-IO.print(c is A) // expect: true
-IO.print(c is B) // expect: true
-IO.print(c is C) // expect: true
+System.print(c is A) // expect: true
+System.print(c is B) // expect: true
+System.print(c is C) // expect: true
 
-IO.print(c.aField) // expect: two
-IO.print(c.bField) // expect: one
-IO.print(c.cField) // expect: c
+System.print(c.aField) // expect: two
+System.print(c.bField) // expect: one
+System.print(c.cField) // expect: c