feat: replace IO class with System class and remove separate IO module

- Delete wren_io.c, wren_io.h, and io.wren source files
- Remove multi-arity print overloads and IO.read/IO.time methods
- Add System class with print, printAll, write, writeAll methods to core.wren
- Update all documentation examples and README to use System.print instead of IO.print

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]!