[Rust] Add builder API for creating workflows

`Workflow::new` is replaced by `Workflow::build` that returns a
`Builder` type that supports the operations that mutate a workflow, such
as registering activities.

`Workflow::instance` is replaced by `Workflow::get` to make clear that
it is intended to look up an existing workflow.

`Workflow::cloned` is introduced to wrap the common pattern of
retrieving an existing workflow and cloning it with the same name in
order to modify it.

`Builder::activity_before` / `Builder::activity_after` are introduced to
wrap the common pattern of registering an activity then inserting it
before or after a given activity.

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