Feature Specific Profile Support

Parsers using Parsack can now use feature-specific profilers.

Documentation can be found at: http://pkg-build.racket-lang.org/doc/feature-profile/index.html

Use parsack-features in #:extra-features.
First, this is a fantastic idea!  I'm really glad you're doing this!

One question. Although I'm not very familiar with the `feature-profile` package, and maybe I'm misunderstanding, its docs say there are three types of marks.

> Active marks correspond directly to continuation marks. Feature code can be instrumented with active marks by wrapping it in

>    `(with-continuation-mark feature-key payload body)`

> Active marks are the simplest flavor of feature marks, and do not require recompiling programs (with feature-profile-compile-handler) before profiling. On the other hand, they impose a performance overhead even when not profiling, and must be used with caution.

Are you using an "active mark" here? Is the performance warning relevant? Should this be using a "functional latent mark" instead?

Again, I'm not up-to-speed on racket-profile so I apologize if this is a dumb question. And again, great idea!