subscriber: add Subscribe impl for Vec<S: Subscribe>

[hawkw]

Depends on #2028

Motivation

In many cases, it is desirable to have a variable number of subscribers
at runtime (such as when reading a logging configuration from a config
file). The current approach, where types implementing Subscribe are
composed at the type level into Layered subscribers, doesn't work well
when the number of subscribers varies at runtime.

Solution

To solve this, this branch adds a Subscribe implementation for
Vec<S> where S: Subscribe. This allows variable-length lists of
subscribers to be added to a collector. Although the impl for Vec<S>
requires all the subscribers to be the same type, it can also be used in
conjunction with Box<dyn Subscribe<C> + ...> trait objects to
implement a variable-length list of subscribers of multiple types.

I also wrote a bunch of docs examples.

Notes

Alternatively, we could have a separate type defined in
tracing-subscriber for a variable-length list of type-erased
subscribers. This would have one primary usability benefit, which is
that we could have a push operation that takes an impl Subscribe
and boxes it automatically. However, I thought the approach used
here is nicer, since it's based on composing together existing
primitives such as Vec and Box, rather than adding a whole new API.
Additionally, it allows avoiding the additional Boxing in the case
where the list consists of subscribers that are all the same type.

Closes #1708