"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/limit/long_string.wren

@@ -1,6 +1,6 @@
 // Create a very long string containing escape sequences.
 var s = "1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\n1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\n1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\n1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\n1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\n1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\n1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\n1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\n1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\n1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890\"1234567890"
-IO.print(s)
+System.print(s)
 // expect: 1234567890"1234567890"1234567890"1234567890"1234567890"1234567890"1234567890"1234567890"1234567890"1234567890
 // expect: 1234567890"1234567890"1234567890"1234567890"1234567890"1234567890"1234567890"1234567890"1234567890"1234567890
 // expect: 1234567890"1234567890"1234567890"1234567890"1234567890"1234567890"1234567890"1234567890"1234567890"1234567890

test/limit/many_constants.wren

@@ -65534,7 +65534,7 @@ var f = Fn.new {
   65533
   65534
   65535
-  IO.print(65536)
+  System.print(65536)
 }
 
 f.call() // expect: 65536

test/limit/many_fields.wren

@@ -255,7 +255,7 @@ class Foo {
     _field253 = 253
     _field254 = 254
     _field255 = 255
-    IO.print(_field255)
+    System.print(_field255)
   }
 }
 

test/limit/many_globals.wren

@@ -4095,7 +4095,7 @@ var global4094 = 4094
 var global4095 = 4095
 var global4096 = 4096
 
-IO.print(global4096) // expect: 4096
+System.print(global4096) // expect: 4096
 
 // Symbol tables currently use O(n) to look up names. This makes this test
 // painfully slow when a large number of names are defined. This is a
@@ -65484,5 +65484,5 @@ var global65472 = 65472
 // Not defining the full 65,536 here because the core library also defines some
 // global names.
 
-IO.print(global65472) // expect 65472
+System.print(global65472) // expect 65472
 */

test/limit/many_inherited_fields.wren

@@ -131,8 +131,8 @@ class Foo {
   }
 
   foo {
-    IO.print(_field1)
-    IO.print(_field128)
+    System.print(_field1)
+    System.print(_field128)
   }
 }
 
@@ -269,8 +269,8 @@ class Bar is Foo {
   }
 
   bar {
-    IO.print(_field129)
-    IO.print(_field255)
+    System.print(_field129)
+    System.print(_field255)
   }
 }
 

test/limit/too_many_constants.wren

@@ -65535,5 +65535,5 @@ var f = Fn.new {
   65534
   65535
   66536
-  IO.print(65537) // expect error
+  System.print(65537) // expect error
 }

test/limit/too_many_inherited_fields.wren

@@ -131,8 +131,8 @@ class Foo {
   }
 
   foo {
-    IO.print(_field1)
-    IO.print(_field128)
+    System.print(_field1)
+    System.print(_field128)
   }
 }
 
@@ -270,8 +270,8 @@ class Bar is Foo { // expect runtime error: Class 'Bar' may not have more than 2
   }
 
   bar {
-    IO.print(_field129)
-    IO.print(_field256)
+    System.print(_field129)
+    System.print(_field256)
   }
 }