No functional changes, just changing all the orderfoo functions
into (*Order).foo methods.

Passes toolstash-check.

Change-Id: Ib9833daa98aff3c645ce56794a414f8472689152
Reviewed-on: https://go-review.googlesource.com/98617
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>