optimization: struct alignment

[behnambm]

Pull requset to resolve #2632

Modified files with saved memory size:

File name	Saved size(bytes)
binder.go	40
echo.go	8
echo_fs.go	8
group.go	16
ip.go	8
response.go	16
router.go	32
context.go	16
echo.go	8

Total saved size: 144

There was potential to further optimize the Echo struct in the echo.go file, but I chose to respect the readability of the codebase.

@aldas a quick review from you would be awesome whenever you’re free. Thanks!

[aldas]

LGTM