"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/system/print.wren

@@ -0,0 +1,12 @@
+class Foo {
+  construct new() {}
+
+  toString { "Foo.toString" }
+}
+
+// Calls toString on argument.
+System.print(Foo.new()) // expect: Foo.toString
+
+// Returns the argument.
+System.print(System.print(1) == 1) // expect: 1
+                                   // expect: true

test/core/system/print_all.wren

@@ -0,0 +1,8 @@
+class Foo {
+  construct new() {}
+
+  toString { "Foo" }
+}
+
+System.printAll([]) // expect:
+System.printAll([1, true, Foo.new(), "s"]) // expect: 1trueFoos

test/core/system/print_all_not_sequence.wren

@@ -0,0 +1 @@
+System.printAll(123) // expect runtime error: Num does not implement 'iterate(_)'.

test/core/system/print_bad_to_string.wren

@@ -0,0 +1,6 @@
+class BadToString {
+  construct new() {}
+  toString { 3 }
+}
+
+System.print(BadToString.new()) // expect: [invalid toString]

test/core/system/write_all.wren

@@ -0,0 +1,10 @@
+class Foo {
+  construct new() {}
+
+  toString { "Foo" }
+}
+
+System.writeAll([]) // expect:
+System.print()
+System.writeAll([1, true, Foo.new(), "s"]) // expect: 1trueFoos
+System.print()

test/core/system/write_all_not_sequence.wren

@@ -0,0 +1 @@
+System.writeAll(123) // expect runtime error: Num does not implement 'iterate(_)'.

test/core/system/write_bad_to_string.wren

@@ -0,0 +1,7 @@
+class BadToString {
+  construct new() {}
+  toString { 3 }
+}
+
+System.write(BadToString.new())
+System.print("!") // expect: [invalid toString]!