Rewrite #[derive(Identifiable)] in derives2

The actual derive itself is pretty straightforward (this isn't exactly a
complex trait). Most of the tests were brought over from the macro
tests. I dropped a few that were obviously redundant (that were testing
our struct parser not the derive). I've added a few for things that
weren't working before (and were actually broken) like tuple structs.

This does contain what is technically a breaking change in behavior. The
old implementation would look for the PK fields by field name rather
than column name. I consider this a bug. Everything else links by column
name, and if we actually went by field name, tuple structs couldn't
work.

I didn't add explicit UI tests for the table not being in scope, or for
bad syntax, as those are redundant with AsChangeset. We did have to do a
little bit of dancing to work around
rust-lang/rust#47941 when the PK names cause
an error.

CHANGELOG.md

@@ -54,6 +54,9 @@ for Rust libraries in [RFC #1105](https://github.com/rust-lang/rfcs/blob/master/
   old and new timestamp formats. (The migrations were always run in the correct
   order, this only affects the display logic of `migration list`)
 
+* `#[derive(Identifiable)]` now correctly associates `#[primary_key]` with the
+  column name, not field name.
+
 ## [1.1.1] - 2018-01-16
 
 ### Added

diesel/src/macros/mod.rs

@@ -1143,8 +1143,6 @@ mod static_cond;
 #[macro_use]
 mod ops;
 
-#[macro_use]
-mod identifiable;
 #[macro_use]
 mod insertable;