Proposal: add tuple destructuring definition in LET/IN blocks

[ahelwer]

Often I will have some function handling a tuple as an intermediate result. It would be nice to have some way to destructure it instead of having to use indices. It makes sense to add this as an additional definition type inside LET/IN blocks, like:

LET
  result == f[x]
  <<y, z>> == result
IN expr

This would semantically decompose to:

LET
  result == f[x]
  y == result[1]
  z == result[2]
IN expr

Tuple destructuring is present in set map/filter syntax, which is convenient, but it would be nice to have it when dealing with only a single value.