Allow for keyword function mocking

[philomates]

Spent some more time looking at how provided handles mocks with nested expressions and realized that it isn't handling keyword functions (i.e (:a-num {:a-num 1})), as brought up in #389.

Currently when an expression is used as an argument in a provided statement, Midje does not evaluate that expression, but rather only mocks function calls with those that exact expression as an argument. For example:

(def my-inc inc)
(fact "Able to mock exact nested function invocations"
  (f (my-inc 1)) => 2
  (provided
    (f (my-inc 1)) => 2))

works, but if we do some partial evaluation in our test statement, the test fails:

(fact "Unable to handle partial evaluation of mocked functions"
  (f 2) => 2
  (provided
    (f (my-inc 1)) => 2))

This is a trade-off in test framework expressiveness that makes sense.

When we apply this to keyword functions, Midje fails here due to the fact that the fnref classification function doesn't handle keywords.
This prevents us from doing things like

(unfinished f)
(fact "Able to mock exact keyword accessor function invocations"
  (f (:fst {:fst 2})) => 2
  (provided
    (f (:fst {:fst 2})) => 2))

Addresses #362 and #281

[marick]

I originally had some notion of separating out data access (via keywords) from function calls. That was probably a bad idea; certainly it didn't get used.

https://github.com/marick/Midje/wiki/Data-Prerequisites

I don't have an opinion about how to handle keywords-as-functions.