Better unions

[Tinche]

Flesh out our union support, for a start.

[codecov]

Codecov Report

Merging #115 (b788cc8) into master (fbdb947) will increase coverage by 0.50%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #115      +/-   ##
==========================================
+ Coverage   97.72%   98.23%   +0.50%     
==========================================
  Files           7        7              
  Lines         440      453      +13     
==========================================
+ Hits          430      445      +15     
+ Misses         10        8       -2     

Impacted Files	Coverage Δ
src/cattr/_compat.py	100.00% <100.00%> (ø)
src/cattr/converters.py	99.01% <100.00%> (+0.05%)	⬆️
src/cattr/function_dispatch.py	100.00% <100.00%> (+11.76%)	⬆️
src/cattr/gen.py	93.93% <100.00%> (+0.12%)	⬆️
src/cattr/multistrategy_dispatch.py	100.00% <100.00%> (ø)

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