* group load command structs.
* use hex literal for LoadCommand.
  Decimal number is not a proper representation for some commands.
  (e.g. LC_RPATH = 0x8000001c)
* move Symbol struct from macho.go to file.go.
  Symbol is a high level representation, not in Mach-O.

Change-Id: I3c69923cb464fb1211f2e766c02e1b537e0b5de2
Reviewed-on: https://go-review.googlesource.com/56130Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>