"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/if/dangling_else.wren

@@ -1,3 +1,3 @@
 // A dangling else binds to the right-most if.
-if (true) if (false) IO.print("bad") else IO.print("good") // expect: good
-if (false) if (true) IO.print("bad") else IO.print("bad")
+if (true) if (false) System.print("bad") else System.print("good") // expect: good
+if (false) if (true) System.print("bad") else System.print("bad")

test/language/if/else.wren

@@ -1,6 +1,6 @@
 // Evaluate the 'else' expression if the condition is false.
-if (true) IO.print("good") else IO.print("bad") // expect: good
-if (false) IO.print("bad") else IO.print("good") // expect: good
+if (true) System.print("good") else System.print("bad") // expect: good
+if (false) System.print("bad") else System.print("good") // expect: good
 
 // Allow block body.
-if (false) null else { IO.print("block") } // expect: block
+if (false) null else { System.print("block") } // expect: block

test/language/if/if.wren

@@ -1,10 +1,10 @@
 // Evaluate the 'then' expression if the condition is true.
-if (true) IO.print("good") // expect: good
-if (false) IO.print("bad")
+if (true) System.print("good") // expect: good
+if (false) System.print("bad")
 
 // Allow block body.
-if (true) { IO.print("block") } // expect: block
+if (true) { System.print("block") } // expect: block
 
 // Assignment in if condition.
 var a = false
-if (a = true) IO.print(a) // expect: true
+if (a = true) System.print(a) // expect: true

test/language/if/newline_after_if.wren

@@ -1,2 +1,2 @@
 if // expect error
-(true) IO.print("bad")
+(true) System.print("bad")

test/language/if/truth.wren

@@ -1,8 +1,8 @@
 // False and null are false.
-if (false) IO.print("bad") else IO.print("false") // expect: false
-if (null) IO.print("bad") else IO.print("null") // expect: null
+if (false) System.print("bad") else System.print("false") // expect: false
+if (null) System.print("bad") else System.print("null") // expect: null
 
 // Everything else is true.
-if (true) IO.print(true) // expect: true
-if (0) IO.print(0) // expect: 0
-if ("") IO.print("empty") // expect: empty
+if (true) System.print(true) // expect: true
+if (0) System.print(0) // expect: 0
+if ("") System.print("empty") // expect: empty