subscriber: fix reload ergonomics

[hawkw]

Motivation

Currently, the reload layer is generic over both the type of the layer
being reloaded, and the type of the subscriber that the layer is
layered onto. This means that the reload::Handle type that's used to
reload the value of the layer also is parameterized over the
subscriber's type.

The subscriber type parameter makes the reload API significantly
harder to use. Any time a reload::Handle is returned by a function,
taken as an argument, or stored in a struct, the full type of the
subscriber under the layer must be written out --- and often, it is
quite long. What makes this worse is that sometimes the type of the
subscriber may vary at runtime based on some configuration, while the
type of the layer that's reloaded remains the same. For example, in
Linkerd, we've had to do this, which is really not ideal.

Solution

This branch removes the Subscriber type parameter from reload::Layer
and reload::Handle. Now, the Handle type is only generic over the
type of the inner layer that's being reloaded. It turns out that the
Subscriber type parameter was only necessary to add a L: Layer<S>
bound to reload::Layer's constructor, which isn't really necessary ---
if the layer does not implement Layer<S>, the type error will occur
when Subscriber::with is actually used to layer it, which is fine.

I also changed the with_filter_reloading option on the FmtSubscriber
builder to also work with LevelFilters, since there's no reason for it
not to, and added an example.

Since this breaks existing code, this change has to be made as part of
0.3.