subscriber: Performance Improvements to fmt::Layer

[davidbarsky]

(This is followup from #420)

fmt::Layer's performance can probably be improved by pooling allocations of Strings and Extensions. hawkw/natatorium is one possible crate that can be used.

[hawkw]

hawkw/natatorium is one possible crate that can be used.

I think a better solution (rather than trying to get the natatorium stuff releasable) would be to add support for pooling to sharded_slab: hawkw/sharded-slab#2

[davidbarsky]

I believe the registry already makes use of some pooling. I'll close this in the meantime.

[davidbarsky]

Whoops, re-opening. The registry does some pooling, but it doesn't pool strings or hashmaps. it should.