Simplify the structure and codegen of UpdateStatement

[sgrif]

The main goal of this refactoring was to remove the implementations of
SaveChangesDsl from codegen and provide them generically instead.
Since I want the code outside of codegen to be as approachable as
possible, I made a few other changes to reduce the where clause of these
implementations. I still would like to reduce the where clause further,
but this is at a decent place. The following tree of changes were in
support of this goal:

• Identifiable needs to have the table. This is so we can generically do
  table.find(id) for T: Identifiable.
• Structs can now be passed directly to update and delete
  • This caused us to change UpdateTarget to be a struct, and
    introduce an AsUpdateTarget trait so that we can abstract over
    whether the fields on that struct are a reference or not

All of this will drastically simplify the non-procedural-macro form of
#[derive(AsChangeset)], and hopefully lead to more refactoring in the
future.

It should be noted that this changes the query on SQLite from

UPDATE users SET ... WHERE id = 1;
SELECT * FROM users WHERE id = 1 LIMIT 1;

to

UPDATE users SET ... WHERE id = 1;
SELECT * FROM users WHERE id = 1;

I can get the LIMIT 1 back in there if needed, but it makes the where clause more complex, and I don't think it matters to keep that here. I couldn't simplify it in the way I wanted to by pulling first into a separate trait because of rust-lang/rust#34260

[sgrif]

r? @diesel-rs/contributors

[fables-tales]

LGTM