core/api-spec: new api spec
core/api-spec: new api spec

This is only the beginning for the API spec.
We want to be more precise with our spec, but this serves as a great starting point.

Closes chain/chainprv#1168.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@kr




chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain