Improve derive-Display trait bounds inference (#93)

[ffuugoo]

First half of #93.

Modified Display macro, so that for each field of the following types or of reference to
such types a where clause is generated bounding field's type by Display trait:

• T
• T::AssociatedType and <T as Trait>::AssociatedType
• ::path::SomeOtherType<T> and ::path::SomeOtherType<&T>

E.g., for a structure Foo defined like this:

#[derive(Display)]
struct Foo<'a, T1, T2: Trait, T3> {
    field1: T1,
    field2: <T2 as Trait>::Type,
    field3: Bar<T3>,
    field4: &'a T1,
    field5: &'a <T2 as Trait>::Type,
    field6: &'a Bar<T3>,
}

Following where clauses would be generated (notice, that for reference types the
referred type is bound):

• T1: Display
• <T2 as Trait>::Type: Display
• Bar<T3>: Display

As far as I understand, as we can't define Display for pointers, arrays, slices and tuples, adding bounds for such types (or any generic types parametrized by such types, e.g., SomeType<[T; 10]>) would be wrong.

[tyranron]

Seems nice! 👍

Tested on a real project, works as expected.

[ffuugoo]

Implemented proper trait bounds inference for reference types as suggested by @tyranron in discussion in #93.

[JelteF]

Can you add a test for the reference types?

[ffuugoo]

@JelteF

[JelteF]

Thanks for the work! I'll take a look at it in the near future

…

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[JelteF]

This is now released as version 0.99.0. Announcement link: https://www.reddit.com/r/rust/comments/duptc0/derive_more_0990_released_biggest_release_ever/