Migrate doc tests to use macros 1.1

[killercup]

Since macros 1.1 is the way to use diesel from 0.10/rust 1.15 forward, we should migrate to using custom derives in our API documentation, instead of the macros. This will introduce a (dev) dependency on diesel_codegen, but in general it shouldn't be a problem.

With a quick rg '///(.*?)impl_(\w+)!' -g '!src/macros' I could only find 4 usages of the impl_* macros.

[dstu]

What do you recommend doing to ensure that doctests are working properly? What is the best way to run modified doctests to ensure that they work properly? cargo test, bin/test, or something else?

[killercup]

Good question, @dstu! I'm a lazy person, so I don't want to wait for all the tests to run! :) When I added the doc test to #561, I used:

$ pwd # in diesel crate dir
…/diesel/diesel
$ env RUST_BACKTRACE=1 DATABASE_URL=postgresql://pascal@localhost/diesel_test DATABASE_URL_FOR_SCHEMA=postgresql://pascal@localhost/diesel_schema cargo +nightly test --no-default-features --features "postgres serde_json unstable" -- Jsonb

The important parts are the features, and the -- <whatever> after the cargo test arguments, as that tells the test runner to only execute the tests whose names contain the <whatever>.

If you are happy with your changes and your new/changed tests run, you can run the whole test suite. Or, you know, let Travis to it when you open a PR ;)