## Motivation

Currently we have a lot of implementations in `linera-views` that don't
work on the Web because they require `Sync` bounds. For example,
`MapView` and `QueueView` are not usable on the Web.

## Proposal

Conditionally use `trait-variant` to add bounds to the traits themselves
rather than the implementations. [Futures should ‘always’ be
`Sync`](https://internals.rust-lang.org/t/what-shall-sync-mean-across-an-await/12020/18)
anyway.

`linera_storage_service::client` contains an implementation of
`KeyValueStore` that is not `Sync` even though it isn't on the Web, due
to a trait object inside `tonic` that
[has](hyperium/tonic#81)
[caused](hyperium/tonic#82)
[some](hyperium/tonic#84)
[contention](hyperium/tonic#117). For that
implementation, follow the solution [given
there](hyperium/tonic#117 (comment))
and use
[`sync_wrapper::SyncFuture`](https://docs.rs/sync_wrapper/latest/sync_wrapper/struct.SyncFuture.html)
to make the futures `Sync` based on the fact that they can only be used
through `Pin<&mut Self>` anyway.

Ditto `aws-smithy-async`.

## Test Plan

CI.

## Release Plan

- Nothing to do / These changes follow the usual release cycle.

## Links

- [reviewer
checklist](https://github.com/linera-io/linera-protocol/blob/main/CONTRIBUTING.md#reviewer-checklist)