"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/core/sequence/all.wren

@@ -1,10 +1,10 @@
 var a = [1, 2, 3]
-IO.print(a.all {|x| x > 1 }) // expect: false
-IO.print(a.all {|x| x > 0 }) // expect: true
-IO.print([].all {|x| false }) // expect: true
+System.print(a.all {|x| x > 1 }) // expect: false
+System.print(a.all {|x| x > 0 }) // expect: true
+System.print([].all {|x| false }) // expect: true
 
 // Returns first falsey value.
-IO.print(a.all {|x| x < 2 ? null : false }) // expect: null
+System.print(a.all {|x| x < 2 ? null : false }) // expect: null
 
 // Returns last truthy value.
-IO.print(a.all {|x| x }) // expect: 3
+System.print(a.all {|x| x }) // expect: 3

test/core/sequence/any.wren

@@ -1,10 +1,10 @@
 var a = [1, 2, 3]
-IO.print(a.any {|x| x > 3 }) // expect: false
-IO.print(a.any {|x| x > 1 }) // expect: true
-IO.print([].any {|x| true }) // expect: false
+System.print(a.any {|x| x > 3 }) // expect: false
+System.print(a.any {|x| x > 1 }) // expect: true
+System.print([].any {|x| true }) // expect: false
 
 // Returns first truthy value.
-IO.print(a.any {|x| x }) // expect: 1
+System.print(a.any {|x| x }) // expect: 1
 
 // Returns last falsey value.
-IO.print(a.any {|x| x < 2 ? null : false }) // expect: false
+System.print(a.any {|x| x < 2 ? null : false }) // expect: false

test/core/sequence/count.wren

@@ -10,4 +10,4 @@ class TestSequence is Sequence {
   iteratorValue(iterator) { iterator }
 }
 
-IO.print(TestSequence.new().count) // expect: 10
+System.print(TestSequence.new().count) // expect: 10

test/core/sequence/is_empty.wren

@@ -1,5 +1,5 @@
-IO.print([].isEmpty) // expect: true
-IO.print([1].isEmpty) // expect: false
+System.print([].isEmpty) // expect: true
+System.print([1].isEmpty) // expect: false
 
 class InfiniteSequence is Sequence {
   construct new() {}
@@ -8,4 +8,4 @@ class InfiniteSequence is Sequence {
 }
 
 // Should not try to iterate the whole sequence.
-IO.print(InfiniteSequence.new().isEmpty) // expect: false
+System.print(InfiniteSequence.new().isEmpty) // expect: false

test/core/sequence/map.wren

@@ -29,20 +29,20 @@ class Fib is Sequence {
 var squareFib = Fib.new().map {|fib| fib * fib }
 var iterator = null
 
-IO.print(squareFib is Sequence) // expect: true
-IO.print(squareFib) // expect: instance of MapSequence
+System.print(squareFib is Sequence) // expect: true
+System.print(squareFib) // expect: instance of MapSequence
 
 iterator = squareFib.iterate(iterator)
-IO.print(squareFib.iteratorValue(iterator)) // expect: 0
+System.print(squareFib.iteratorValue(iterator)) // expect: 0
 
 iterator = squareFib.iterate(iterator)
-IO.print(squareFib.iteratorValue(iterator)) // expect: 1
+System.print(squareFib.iteratorValue(iterator)) // expect: 1
 
 iterator = squareFib.iterate(iterator)
-IO.print(squareFib.iteratorValue(iterator)) // expect: 1
+System.print(squareFib.iteratorValue(iterator)) // expect: 1
 
 iterator = squareFib.iterate(iterator)
-IO.print(squareFib.iteratorValue(iterator)) // expect: 4
+System.print(squareFib.iteratorValue(iterator)) // expect: 4
 
 iterator = squareFib.iterate(iterator)
-IO.print(squareFib.iteratorValue(iterator)) // expect: 9
+System.print(squareFib.iteratorValue(iterator)) // expect: 9

test/core/sequence/to_list.wren

@@ -10,4 +10,4 @@ class TestSequence is Sequence {
   iteratorValue(iterator) { iterator }
 }
 
-IO.print(TestSequence.new().toList) // expect: [1, 2, 3]
+System.print(TestSequence.new().toList) // expect: [1, 2, 3]

test/core/sequence/where.wren

@@ -29,14 +29,14 @@ class Fib is Sequence {
 var largeFibs = Fib.new().where {|fib| fib > 100 }
 var iterator = null
 
-IO.print(largeFibs is Sequence) // expect: true
-IO.print(largeFibs) // expect: instance of WhereSequence
+System.print(largeFibs is Sequence) // expect: true
+System.print(largeFibs) // expect: instance of WhereSequence
 
 iterator = largeFibs.iterate(iterator)
-IO.print(largeFibs.iteratorValue(iterator)) // expect: 144
+System.print(largeFibs.iteratorValue(iterator)) // expect: 144
 
 iterator = largeFibs.iterate(iterator)
-IO.print(largeFibs.iteratorValue(iterator)) // expect: 233
+System.print(largeFibs.iteratorValue(iterator)) // expect: 233
 
 iterator = largeFibs.iterate(iterator)
-IO.print(largeFibs.iteratorValue(iterator)) // expect: 377
+System.print(largeFibs.iteratorValue(iterator)) // expect: 377