"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/io/print.wren

@@ -1,34 +0,0 @@
-class Foo {
-  construct new() {}
-
-  toString { "Foo.toString" }
-}
-
-// Calls toString on argument.
-IO.print(Foo.new()) // expect: Foo.toString
-
-// With one argument, returns the argument.
-IO.print(IO.print(1) == 1) // expect: 1
-                           // expect: true
-
-// With more than one argument, returns null.
-IO.print(IO.print(1, 2) == null) // expect: 12
-                                 // expect: true
-
-// Allows multiple arguments and concatenates them.
-IO.print(1) // expect: 1
-IO.print(1, 2) // expect: 12
-IO.print(1, 2, 3) // expect: 123
-IO.print(1, 2, 3, 4) // expect: 1234
-IO.print(1, 2, 3, 4, 5) // expect: 12345
-IO.print(1, 2, 3, 4, 5, 6) // expect: 123456
-IO.print(1, 2, 3, 4, 5, 6, 7) // expect: 1234567
-IO.print(1, 2, 3, 4, 5, 6, 7, 8) // expect: 12345678
-IO.print(1, 2, 3, 4, 5, 6, 7, 8, 9) // expect: 123456789
-IO.print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10) // expect: 12345678910
-IO.print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11) // expect: 1234567891011
-IO.print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12) // expect: 123456789101112
-IO.print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13) // expect: 12345678910111213
-IO.print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14) // expect: 1234567891011121314
-IO.print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15) // expect: 123456789101112131415
-IO.print(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16) // expect: 12345678910111213141516

test/io/print_bad_to_string.wren

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

test/io/read.wren

@@ -1,13 +0,0 @@
-var a = IO.read("a:") // stdin: first
-var b = IO.read("b:") // stdin: second
-IO.print
-// Using write() here because the read lines include the trailing newline.
-IO.write(a)
-IO.write(b)
-
-// Since stdin isn't echoed back to stdout, we don't see the input lines here,
-// and there is no newline between the two prompts since that normally comes
-// from the input itself.
-// expect: a:b:
-// expect: first
-// expect: second

test/io/read_arg_not_string.wren

@@ -1 +0,0 @@
-IO.read(null) // expect runtime error: Prompt must be a string.

test/io/read_eof.wren

@@ -1 +0,0 @@
-IO.write(IO.read()) // expect: null

test/io/write_bad_to_string.wren

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