Add a block_log_util that allows you compare block log files.

Currently it supports three operations:

• sha256sum: compute/verify the hash of a block log as if it were stored uncompressed. this lets you compare hashes of block logs using different compression settings, without having to decompress the logs first
• cmp: directly compare two block logs, as if they were both uncompressed. this will return success if both logs represent the same blocks, even if compressed differently
• truncate: remove blocks from the end of a block log
• Change the fc::sha256 implementation to use the EVP api instead of the deprecated SHA256_* methods, and allow sha256 encoders to be copied.
• Add a block_log::for_each_block() method that walks over the specified block range. The original method of that name, which was only used for rebuilding the artifacts file, was renamed for_each_block_reverse.