Note [Floating Ticks in CorePrep]

It might seem counter-intuitive to float ticks by default, given
that we don't actually want to move them if we can help it. On the
other hand, nothing gets very far in CorePrep anyway, and we want
to preserve the order of let bindings and tick annotations in
relation to each other. For example, if we just wrapped let floats
when they pass through ticks, we might end up performing the
following transformation:

  src<...> let foo = bar in baz
  ==>  let foo = src<...> bar in src<...> baz

Because the let-binding would float through the tick, and then
immediately materialize, achieving nothing but decreasing tick
accuracy. The only special case is the following scenario:

  let foo = src<...> (let a = b in bar) in baz
  ==>  let foo = src<...> bar; a = src<...> b in baz

Here we would not want the source tick to end up covering "baz" and
therefore refrain from pushing ticks outside. Instead, we copy them
into the floating binds (here "a") in cpePair. Note that where "b"
or "bar" are (value) lambdas we have to push the annotations
further inside in order to uphold our rules.

All of this is implemented below in @wrapTicks@.