allow _ as the fallback 'wildcard' case

[MasonProtter]

Closes #55

With this change, we can now do

julia> is_apple(f::Fruit) = @cases f begin
           apple => true
           _ => false
       end
is_apple (generic function with 1 method)

julia> is_apple(apple)
true

julia> is_apple(mango)
false

julia> code_typed(is_apple, Tuple{Fruit})
1-element Vector{Any}:
 CodeInfo(
1 ─ %1 = (getfield)(f, :data)::Union{SumTypes.Variant{:apple, (), Tuple{}}, SumTypes.Variant{:banana, (), Tuple{}}, SumTypes.Variant{:orange, (), Tuple{}}, SumTypes.Variant{:kiwi, (), Tuple{}}, SumTypes.Variant{:pear, (), Tuple{}}, SumTypes.Variant{:mango, (), Tuple{}}}
│   %2 = (%1 isa SumTypes.Variant{:apple})::Bool
└──      goto #3 if not %2
2 ─      return true
3 ─      return false
) => Bool

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (003c9d4) 100.00% compared to head (c6e15fc) 100.00%.

❗ Current head c6e15fc differs from pull request most recent head 9541e58. Consider uploading reports for the commit 9541e58 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##            master       #56   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines          249       260   +11     
=========================================
+ Hits           249       260   +11     

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