API v1.0

[elshize]

Along with the index format (#143) we need to clean up and stabilize our programmatic API.

I vote to do it via function signatures and concepts for types. Although concepts are not yet in the standard only supported by GCC, we can still use them as a formal description of the types we accept. We can also optionally use -fconcepts feature for gcc, and have a build on Travis, which would check their correctness.

• Block codecs Block Codec concept #250
• Cursors Cursor concept discussion #251
• Index Index concept discussion #252