Optimize writeElement.

This commit modifies the writeElement function to have a "fast path" which uses type
assertions for all of the types which btcwire write so the more expensive
reflection-based binary.Write can be avoided.

Also, this changes all cases that were writing raw ShaHash (32-byte) arrays (which
requires a stack copy) instead simply passing the pointer.

The following benchmark results show the results for serializing a block header
after these changes:

Before: BenchmarkWriteBlockHeader         500000              5566 ns/op
After:  BenchmarkWriteBlockHeader        1000000               991 ns/op

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