Replace gorilla/mux with http.ServeMux path patterns

[benhoyt]

Now that Go's own net/http.ServeMux has path pattern matching, we can get rid of the gorilla/mux dependency. This will probably reduce binary size a little and also likely increase performance a little. We should do a proof of concept of this in Pebble, but propose to do this upstream in snapd too (or first?).

[benhoyt]

I've got a draft of this in PR #684.

[benhoyt]

This is relatively easy to do in Pebble (see draft PR above), but it's going to be a bunch more work to coordinate with other projects such as snapd. We should at least see whether it's going to work in snapd and if so we can point them to this PR and they can do the same thing (we don't necessarily need to do the work for them).