- context: `end()` methods need to consume self so that the user must
  not use the contextual object after having emitted the end artifact.
  However, this introduces an issue in `scope()` methods, in that we
  need to pass the run object both to the async lambda and to call end,
  which forces the lambda to capture by reference. We cannot move into
  the lambda because we need to consume self in `end()`, so the lambda
  must receive a reference. This leads to needing to use a boxed future,
  because rust syntax doesn't have a way to specify captured lifetimes
  in async anon.
- since `end()` is the only contentious method, make a new type
  `ScopedTestRun` which has everything except `end` and pass that with
  lifetime 'static into the lambda. This removes the need for the boxed
  future since no shorter refs are captured.

Signed-off-by: mimir-d <[email protected]>

- similar to previous commit, this removes the dependency on boxed
  futures; remove the cargo dependency as well, and the feature flag

Signed-off-by: mimir-d <[email protected]>

- this commit adds template args such that the users dont need to
  manually covert their values into `tv::Value`
- applies for measurements, metadata and run start params

Signed-off-by: mimir-d <[email protected]>

- fix readme to remove boxed-scopes

Signed-off-by: mimir-d <[email protected]>