Reduces process-wide heap (inuse_space) by 60kB by using a pointer to
a fixed-sized array instead of a slice of a fixed size.

Before:
119.44kB 23.43% 23.43%   147.88kB 29.01%  golang.org/x/net/http2/hpack.addDecoderNode

After:
 59.72kB 13.28% 39.85%    87.94kB 19.56%  golang.org/x/net/http2/hpack.addDecoderNode

(This is all work from an init func in http2/hpack)

Doesn't seem to affect runtime performance.

Measured with:

$ cat huffman_test.go
package main

import (
        "testing"

        _ "golang.org/x/net/http2"
        )

func TestMem(t *testing.T) {}

$ GODEBUG=memprofilerate=1 go test -memprofilerate=1 -memprofile=mem.prof -v .
=== RUN   TestMem
--- PASS: TestMem (0.00s)
PASS
ok      huffmem 0.052s

$ go tool pprof --inuse_space mem.prof

Change-Id: I5e56a5a2682f1063c955b342b37e97ca4c303dab
Reviewed-on: https://go-review.googlesource.com/127235Reviewed-by: Ian Lance Taylor <iant@golang.org>