Reading files!

- File.close()
- File.open()
- File.read()
- file.readBytes()

And a few other little methods. Still lots more work to do, but it's a
start.

test/io/file/open_block_nonexistent.wren

@@ -0,0 +1,3 @@
+import "io" for File
+
+File.open("nonexistent") {|file|} // expect runtime error: no such file or directory