Method calls on interfaces with large stored values
will call the pointer receiver method which may be
a wrapper over a method with value receiver.

This is particularly inefficient for very small bodies.
Inlining the wrapped method body saves a potentially expensive
function call.

benchmark                old ns/op    new ns/op    delta
BenchmarkSortString1K       802295       641387  -20.06%
BenchmarkSortInt1K          359914       238234  -33.81%
BenchmarkSortInt64K       35764226     22803078  -36.24%

Fixes #4707.

R=golang-dev, daniel.morsing, rsc
CC=golang-dev
https://golang.org/cl/7214044