Adds serialization microbenchmark

[GambolingPangolin]

This patch adds microbenchmarks covering

• Encoding/decoding
• binary & cereal
• blocks and transactions

For blocks, I chose a few recent blocks at random. Same with transactions.

[GambolingPangolin]

Benchmark results: master_bench.txt

Generally cereal has worse performance than binary for these examples.

[ProofOfKeags]

I see no reason we can't merge this immediately.