Provide ConstructQuery and ConstructMutation to public interface

[Antipov-Anton]

I need this feature to write logs with full request data.

It's easy and beautiful to use your methods as library public interface.

[dmitshur]

Thanks for the PR. This is very related to the issue shurcooL/githubv4#36.

I would like to find a way to support what you're trying to do (perform logging or tracing, and be able to include information such as the GraphQL query that was sent to remote server for execution), but I haven't found a satisfactory API to make it possible yet.

You're welcome to use this change in your fork/local version or an equivalent solution until shurcooL/githubv4#36 is resolved, but I won't be able to merge this change as is. My goal is to have a high-level and friendly API in this package, and these two functions are too low level. Let's discuss this further and look for a viable API in shurcooL/githubv4#36; then it can be implemented after we agree on a design.

I'll close this PR for now. Thanks again.