ts: better use of `imp` & `elem` args in `Pad{Sink,Src}Handler`s

This is a follow-up to commit 7ee4afac.

This commit cleans up the `Pad{Sink,Src}Handler` by

- Keeping arguments which are strictly necessary.
- Passing arguments by value for the trait functions which return
  a `Future`. The arguments which were previously passed by reference
  were `clone`d internally and then `clone`d again in most
  implementations.

There are unfortunate differences in trait function signatures
between those which return a `Future` and the sync functions. This
is due to the requirement for the arguments to be moved to the
resulting `Future`, whereas sync functions can rely on references.
One particular notable difference is the use of the `imp` in sync
functions instead of the `elem` in functions returning a `Future`.
Because the `imp` is not guaranteed to implement `Clone`, we can't
move it to the resulting `Future`, so the `elem` is used.

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