[conformal/btcwire] a9183f: Ensure all serialization is using fast path.

[Dave Collins]

  Branch: refs/heads/master
  Home:   https://github.com/conformal/btcwire
  Commit: a9183f688ff8134c0985c4196eb38aca0da49001
      
https://github.com/conformal/btcwire/commit/a9183f688ff8134c0985c4196eb38aca0da49001
  Author: Dave Collins <da...@conformal.com>
  Date:   2015-01-14 (Wed, 14 Jan 2015)

  Changed paths:
    M msgalert.go
    M msgmerkleblock.go

  Log Message:
  -----------
  Ensure all serialization is using fast path.

The writeElement function provides faster serialization for primitives.
This commit modifies all instances that call writeElement with a
pointer to a primitive or a byte slice to instead use the primitive /
writeVarBytes function so the faster serialization paths are used.