instrument's new option for adding :depth to execution logs

[athos]

It would sometimes be useful if each log item in instrument's execution logs had an additional key that indicates their nesting level (depth) of function calls at invocation time, especially when debugging a recursive function.

This PR adds another option {:with-depth true} for optionally adding such a key to each log item in the execution logs, like the following:

(require '[postmortem.core :as pm]
         '[postmortem.instrument :as pi])

(defn fact [n]
  (if (= n 0)
    1
    (* n (fact (dec n)))))

;; without :with-depth option
(pi/instrument `fact)
(fact 5) ;=> 120
(pm/log-for `fact)
;=> [{:args (5)}
;    {:args (4)}
;    {:args (3)}
;    {:args (2)}
;    {:args (1)}
;    {:args (0)}
;    {:args (0), :ret 1}
;    {:args (1), :ret 1}
;    {:args (2), :ret 2}
;    {:args (3), :ret 6}
;    {:args (4), :ret 24}
;    {:args (5), :ret 120}]

(pm/reset!)

;; with :with-depth option
(pi/instrument `fact {:with-depth true})
(fact 5) ;=> 120
(pm/log-for `fact)
;=> [{:args (5), :depth 1}
;    {:args (4), :depth 2}
;    {:args (3), :depth 3}
;    {:args (2), :depth 4}
;    {:args (1), :depth 5}
;    {:args (0), :depth 6}
;    {:args (0), :ret 1, :depth 6}
;    {:args (1), :ret 1, :depth 5}
;    {:args (2), :ret 2, :depth 4}
;    {:args (3), :ret 6, :depth 3}
;    {:args (4), :ret 24, :depth 2}
;    {:args (5), :ret 120, :depth 1}]

[codecov]

Codecov Report

Merging #6 into master will increase coverage by 0.26%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #6      +/-   ##
==========================================
+ Coverage   91.71%   91.97%   +0.26%     
==========================================
  Files           7        7              
  Lines         338      349      +11     
  Branches       17       17              
==========================================
+ Hits          310      321      +11     
  Misses         11       11              
  Partials       17       17

Impacted Files	Coverage Δ
src/postmortem/instrument/core.cljc	97.72% <100%> (+0.75%)	⬆️

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