Refactor TS declaration files

[danielrearden]

This PR moves the type declarations inside src and splits them up by file. It also moves tsconfig.ts into the project root and makes some necessary changes to keep dtslint working.

[coveralls]

Coverage remained the same at 99.88% when pulling 9df7fa8 on danielrearden:refactor-typings into 0f74f05 on graphql:master.

[IvanGoncharov]

@danielrearden I need to think about dtslint and I will try to find correct solution in graphql-js so you can latter port it here.

Meanwhile, it would be great if you can ensure that we have types on all function arguments (except inline functions and functions defined as object properties) since TS will require us to define it anyway but TS conversion diff will be way smaller.
You can try to find some set of options that will force ESLint to detect such places (not really necessary here but we can use this approach for larger code base in graphql-js).
Importantly don't type functions unless required by TS.

[IvanGoncharov]

@danielrearden I merged #637 and it should unblock this PR.
Can you please rebase?

[danielrearden]

@IvanGoncharov Rebased!

[IvanGoncharov]

@danielrearden Left a couple of comments, everything else looks good 👍