unicode/utf16: speed up and clean up Encode and EncodeRune

unicode/utf16: speed up and clean up Encode and EncodeRune

name                        old time/op  new time/op  delta
EncodeValidASCII-4          74.1ns ± 1%  70.1ns ± 1%   -5.46%  (p=0.000 n=10+10)
EncodeValidJapaneseChars-4  61.3ns ± 0%  58.9ns ± 0%   -3.82%  (p=0.000 n=10+10)
EncodeRune-4                13.1ns ± 1%   9.8ns ± 0%  -25.24%   (p=0.000 n=10+9)

Fixes #6957

Change-Id: I9dde6d77420c34c6e2ef3e6213bb6be9b58a3074
Reviewed-on: https://go-review.googlesource.com/19891
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Rob Pike <r@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>