feat(codegen): Inhibit generic bounds if skip_serializing

[dtolnay]

Addresses part of #197.

The generated code for a struct like:

struct Test<A, B, C> {
    a: X<A>,
    #[serde(skip_serializing)]
    b: B,
    #[serde(serialize_with="...")]
    c: C,
}

Used to be:

impl<A, B, C> Serialize for Test<A, B, C>
    where A: Serialize,
          B: Serialize,
          C: Serialize,
{ ... }

Now it is:

impl<A, B, C> Serialize for Test<A, B, C>
    where X<A>: Serialize,
{ ... }

Both skip_serializing and serialize_with mean the type does not need to implement Serialize.

The remaining work for #197 is to add a skip_deserializing attribute that does the equivalent for Deserialize (also filed as #259), and to infer T: Default appropriately.

[dtolnay]

It looks like this works on beta and nightly but not stable because it depends on rust-lang/rust#30389. I will need to either omit bounds for types that do not involve type parameters, or revert to the old behavior on <1.7.0.

[dtolnay]

I have added additional filtering to make this work with <1.7.0.

[erickt]

@dtolnay: This was awesome, thanks!