[conformal/btcwire] 8a1828: Optimize writeVarString.

[Dave Collins]

  Branch: refs/heads/master
  Home:   https://github.com/conformal/btcwire
  Commit: 8a1828a2d66cef3d49e034c591d9ed9a140e1f83
      
https://github.com/conformal/btcwire/commit/8a1828a2d66cef3d49e034c591d9ed9a140e1f83
  Author: Dave Collins <da...@conformal.com>
  Date:   2013-11-05 (Tue, 05 Nov 2013)

  Changed paths:
    M common.go

  Log Message:
  -----------
  Optimize writeVarString.

Before:

BenchmarkWriteVarStr4    1000000              1114 ns/op
BenchmarkWriteVarStr10   1000000              1352 ns/op

After:

BenchmarkWriteVarStr4    5000000               291 ns/op
BenchmarkWriteVarStr10  10000000               248 ns/op

This is part ef the ongoing effort to optimize serialization as noted in
conformal/btcd#27.