Giving them specific types has the benefit that
binary.BigEndian.Uint32(b) is now a direct call, not an
indirect via a mutable interface value, so it can potentially
be inlined.

Recent changes to the spec relaxed the rules for comparison,
so this code is still valid:

	func isLittle(o binary.ByteOrder) { return o == binary.LittleEndian }

The change does break this potential idiom:

	o := binary.BigEndian
	if foo {
		o = binary.LittleEndian
	}

That must rewrite to give o an explicit binary.ByteOrder type.
On balance I think the benefit from the direct call and inlining
outweigh the cost of breaking that idiom.

R=r, r2
CC=golang-dev
https://golang.org/cl/2427042