Support defmethod argcount validation

[camsaul]

Resolves #59

This is implemented as a separate thing you have to opt-in to rather than using :arglists directly so we don't blow up in cases where people are using a human-friendly arglist like [x y? & {:as options]].

:defmethod-arities

A set of allowed/required arities that defmethod forms are allowed to have. defmethod forms must have arities that
match all of the specified :defmethod-arities, and all of its arities must be allowed by :defmethod-arities:

(m/defmulti ^:private mf
  {:arglists '([x]), :defmethod-arities #{1}}
  keyword)

(m/defmethod mf :x [x] x)
;; => ok

(m/defmethod mf :x ([x] x) ([x y] x y))
;; => error: {:arities {:disallowed #{2}}}

(m/defmethod mf :x [x y] x y)
;; => error: {:arities {:required #{1}}}


(m/defmethod mf :x [x y] x)
;; => error: {:arities {:required #{1 [:>= 3]}, :disallowed #{2}}}

:defmethod-arities must be a set of either integers or [:> n] forms to represent arities with & rest
arguments, e.g. [:>= 3] to mean an arity of three or-more arguments:

;; methods must both a 1-arity and a 3+-arity
(m/defmulti ^:private mf
  {:arglists '([x] [x y z & more]), :defmethod-arities #{1 [:>= 3]}}
  keyword)

(m/defmethod mf :x ([x] x) ([x y z & more] x))
;; => ok

When rest-argument arities are used, Methodical is smart enough to allow them when appropriate even if they do not
specifically match an arity specified in :defmethod-arities:

(m/defmulti ^:private mf
  {:arglists '([x y z & more]), :defmethod-arities #{[:>= 3]}}
  keyword)

(m/defmethod mf :x
  ([a b c] x)
  ([a b c d] x)
  ([a b c d & more] x))
;; => ok, because everything required by [:>= 3] is covered, and everything present is allowed by [:>= 3]

[codecov]

Codecov Report

Base: 86.76% // Head: 87.09% // Increases project coverage by +0.32% 🎉

Coverage data is based on head (59963d9) compared to base (47381ad).
Patch coverage: 95.65% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #130      +/-   ##
==========================================
+ Coverage   86.76%   87.09%   +0.32%     
==========================================
  Files          21       22       +1     
  Lines        1194     1240      +46     
  Branches       66       68       +2     
==========================================
+ Hits         1036     1080      +44     
  Misses         92       92              
- Partials       66       68       +2     

Impacted Files	Coverage Δ
src/methodical/macros/validate_arities.clj	95.65% <95.65%> (ø)

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