Compactify storage

[MasonProtter]

This makes SumTypes smaller. They should now only take up however much room the largest variant needed, plus a discriminator byte, plus alignment bits.

julia> @sum_type Num begin
           Int(::Int)
           Float(::Float64)
           Complex(::Float64,::Float64)
       end hide_variants=true;

before

julia> sizeof(full_type(Num))
56

After

julia> sizeof(full_type(Num))
24

Downside is that the sum types now have extra type parameters relating to the computed storage. They could be eliminated for sum types without parameters, but in general they're necessary unless we get JuliaLang/julia#8472 (comment).

[codecov]

Codecov Report

Merging #20 (a0796be) into master (b0423ba) will increase coverage by 0.81%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           master       #20      +/-   ##
===========================================
+ Coverage   99.18%   100.00%   +0.81%     
===========================================
  Files           3         4       +1     
  Lines         245       307      +62     
===========================================
+ Hits          243       307      +64     
+ Misses          2         0       -2     

Impacted Files	Coverage Δ
src/cases.jl	100.00% <ø> (ø)
src/SumTypes.jl	100.00% <100.00%> (ø)
src/compute_storage.jl	100.00% <100.00%> (ø)
src/sum_type.jl	100.00% <100.00%> (+1.20%)	⬆️

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