feature: sum types & match/case syntax

[mhuesch]

I think match/case is necessary for implementing sum types.

how else can you do case analysis?

(match r
  [ (L l_body) ((+ 1) l_body) ]
  [ (R r_body) 7 ])

^ we might not even have to do any deeper pattern matching - 1 level only?

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migrated from https://github.com/mhuesch/poly-rs/issues/15.