Provide a syntax extension free alternative to #[insertable_into]

[sgrif]

This provides a pure stable alternative to the #[insertable_into]
annotation. The intention is to change the annotation to call this
macro, rather than impl Insertable directly. However, there are some
unaddressed issues for that, and I will submit that as a separate PR to
attempt to keep the PR size reasonable.

The tests for this are a bit messy, as the actual test body doesn't
change much -- Just the struct definition. I moved the most common case
out into a macro, but I opted to just leave the duplication for the
remaining 4-5 cases that didn't fit, instead of trying to make it so dry
it chafes.

We will continue to support syntex as an option on stable, as we can
provide much better error messages from a procedural macro. I would like
to improve the error messages in some cases if possible though (in
particular, we want to handle the case where a unit struct is passed or
where a tuple struct has unannotated fields).

The structure of the macro is intended to be compatible with the
custom_derive crate. This is untested, but will be fully tested once
I've moved all our annotations to stable macros. The goal is for any
struct definition to be copy pasted into this macro, and the macro
parses the struct body to create the proper implementation. For
sufficiently large structs, we can hit the recursion limit, but there's
really no way around that. People will just need to bump the limit.

One case that this macro doesn't handle is when there are annotations
on struct fields other than #[column_name]. I had originally planned
to handle these, but I realized that the only recognized annotation that
could be there on stable is #[cfg], and we are not handling cfg
attributes. We might handle that in the future, but it'd look really
ugly.

Related to #99

[sgrif]

Review? @diesel-rs/core

/cc @aturon @nikomatsakis Seeing some cases of this sort of thing out in the wild is likely relevant to the lang team.

[sgrif]

The build failures are because the code assumes rust-lang/rust#31776. I need to address that, but the changes are minor and shouldn't need to affect review.

[mfpiccolo]

One of the few places where I really appreciate in-code comments. 👍

[sgrif]

Yup. Would normally restructure this code to not need it, but that's basically impossible with this type of macro.

[mfpiccolo]

Macros for days! Nice work Sean. Once travis is happy I am happy. 👍

[tessgriffin]

Documentation seems very reasonable. Looks good to me!