subscriber: make Registry::enter/exit much faster

[hawkw]

Motivation

We've tried very hard to make sure that entering and exiting spans is
lightweight...in the tracing-core core machinery. Unfortunately, we
haven't done any benchmarking of how subscriber implementations actually
handle enter/exit events. It turns out that in tracing-subscriber's
Registry, there's actually significant overhead for entering a span:
calling span.enter() may take as long as 69 ns (on my machine).

Solution

I've written some microbenchmarks for entering and exiting enabled spans
using tracing-subscriber::fmt, comparing them with the overhead of
calling enter on an enabled span. Based on this, I've made some
performance improvements. These optimizations include:

• Removing the HashSet that's used for tracking previously entered
  span IDs, in favor of linear search. Span stacks probably never deep
  enough for a hashmap to be faster than iterating over a couple of
  vec indices.
• Preallocating the vec used for the span stack to hold at least 64
  elements. This means we'll never have a lag spike from reallocating,
  as I think it'll almost never be deeper than 64 IDs.
• Only cloning/dropping an ID's ref count for the first ID in the stack.

This makes entering and exiting enabled spans significantly faster:

It would be nice to continue optimizing this, but this might be about
the best it gets given the other requirements that we're now making
assertions about.

[hawkw]

I'll backport this to v0.1.x when it lands.

[davidbarsky]

I love how tiny this change is and how large the impact is.