Derive `Eq` and `Hash` wherever possible (#2223)

In server SDKs, these traits can be implemented by any shape _except_ if
the shape's closure contains:

1. A `float`, `double`, or `document` shape: floating point types in
   Rust do not implement `Eq`. Similarly, [`document` shapes] may
   contain arbitrary JSON-like data containing floating point values.
2. A [@streaming] shape: all the streaming data would need to be
   buffered first to compare it.

Additionally, the `Hash` trait cannot be implemented by shapes whose
closure contains:

1. A `map` shape: we render `map` shapes as `std::collections::HashMap`,
   which _do not_ implement `Hash`. See
   https://github.com/awslabs/smithy/issues/1567.

In **client SDKs, these traits cannot be derived on any code-generated
Rust types corresponding to Smithy shapes**, since e.g. adding new
optional members to a structure [is a backwards-compatible change], and
doing so alters the semantics of these traits.

However, this commit does implement these traits for the
`aws_smithy_types::date_time::DateTime` and `aws_smithy_types::Blob`
runtime types.

This change is necessary to efficiently implement the `@uniqueItems`
constraint trait in server SDKs.

[`Eq`]: https://doc.rust-lang.org/std/cmp/trait.Eq.html
[`Hash`]: https://doc.rust-lang.org/std/hash/trait.Hash.html
[`document` shapes]: https://smithy.io/2.0/spec/simple-types.html#document
[@streaming]: https://smithy.io/2.0/spec/streaming.html
[is a backwards-compatible change]: https://smithy.io/2.0/guides/evolving-models.html#updating-structures