"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/break/closure_in_for.wren

@@ -1,7 +1,7 @@
 var f
 for (i in [1, 2, 3]) {
   var j = 4
-  f = Fn.new { IO.print(i + j) }
+  f = Fn.new { System.print(i + j) }
   break
 }
 

test/language/break/closure_in_while.wren

@@ -1,7 +1,7 @@
 var f
 while (true) {
   var i = "i"
-  f = Fn.new { IO.print(i) }
+  f = Fn.new { System.print(i) }
   break
 }
 

test/language/break/exit_local_scopes.wren

@@ -1,5 +1,5 @@
 for (i in 0..10) {
-  IO.print(i)
+  System.print(i)
 
   {
     var a = "a"

test/language/break/in_for_loop.wren

@@ -1,7 +1,7 @@
 for (i in [1, 2, 3, 4, 5]) {
-  IO.print(i)
+  System.print(i)
   if (i > 2) break
-  IO.print(i)
+  System.print(i)
 }
 // expect: 1
 // expect: 1

test/language/break/in_while_loop.wren

@@ -1,9 +1,9 @@
 var i = 0
 while (true) {
   i = i + 1
-  IO.print(i)
+  System.print(i)
   if (i > 2) break
-  IO.print(i)
+  System.print(i)
 }
 // expect: 1
 // expect: 1

test/language/break/nested_for_loop.wren

@@ -1,9 +1,9 @@
 for (i in 0..2) {
-  IO.print("outer ", i)
+  System.print("outer " + i.toString)
   if (i > 1) break
 
   for (j in 0..2) {
-    IO.print("inner ", j)
+    System.print("inner " + j.toString)
     if (j > 1) break
   }
 }

test/language/break/nested_while_loop.wren

@@ -1,11 +1,11 @@
 var i = 0
 while (true) {
-  IO.print("outer ", i)
+  System.print("outer " + i.toString)
   if (i > 1) break
 
   var j = 0
   while (true) {
-    IO.print("inner ", j)
+    System.print("inner " + j.toString)
     if (j > 1) break
 
     j = j + 1