Only conditionally enable "less useful" specs

[borkdude]

Related to #44

The following spec aren't really helping when instrumenting, but they do require extra CPU (see https://gist.github.com/borkdude/8a977fa453c6ee26fefcecb4c88fe1f2).

= ;; no way to pass wrong args (except arity)
/ ;; inlined
get ;; no way to pass wrong args (except arity)
some? ;; no way to pass wrong args (except arity)
str ;; no way to pass wrong args

There should be a way have them turned off by default, but to conditionally enable them.

Unstrumenting after the fact would be an option, but I think this should be as smooth as possible for the user of this library, so not loading the fdefs in the first place has my preference.

Moving them to a different namespace would be the easiest option for all environments.

[borkdude]

Fixed in #106.

Running the first 20 coal-mine tests in cljs+node without instrumenting the optional less useful specs, compare

"Elapsed time: 3198.731217 msecs"

to

"Elapsed time: 21343.952922 msecs"

Also note that the tests do not even pass when having the optional specs instrumented due to several "maximum call stack exceeded" errors.