More file IO!

Can now create, delete, and write to files.

test/io/file/create_block.wren

@@ -0,0 +1,25 @@
+import "io" for File
+
+// Create a new file.
+var f
+File.create("file.temp") {|file|
+  f = file
+  System.print(file.isOpen) // expect: true
+  System.print(file.size) // expect: 0
+
+  file.writeBytes("stuff")
+}
+
+// Closed on block exit.
+System.print(f.isOpen) // expect: false
+
+System.print(File.size("file.temp")) // expect: 5
+
+// Overwrite a file.
+File.create("file.temp") {|file|
+  // Truncates.
+  System.print(file.size) // expect: 0
+}
+
+File.delete("file.temp")
+// TODO: Test return value.