fix double-free and compiler crash

[zerbina]

Summary

• fix a double-free issue when assigning to sink parameters of
  instantiated generic tuple type
• fix a compiler crash when passing an empty seq, array, or set
  construction to a generic instantiation receiver type

Fixes #1415.

Details

Post-match fitting didn't consider generic tuple instantiations when
applying implicit conversions, meaning that the nkHiddenSubConv or
nkHiddenStdConv inserted earlier by sigmatch stayed.

This led to two issues:

• nkStmtListExprs of empty container type didn't have their types
  fixed properly, causing crashes when the empty type reached into the
  MIR phase
• a conversion between sink T and T is considered an rvalue
  conversion by proto_mir, which resulted in assignments involving
  these implicit conversions effectively being shallow assignments
  (and thus double frees)

Skipping tyGenericInst and tyAlias (for good measure) makes sure
the implicit conversion is collapsed when the destination type is a
generic instantiation, fixing both issues. A test is added for both.

[saem]

/merge

[github-actions]

Merge requested by: @saem

Contents after the first section break of the PR description has been removed and preserved below:

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Notes for Reviewers

• discovered during mir: lower cast with MIR pass #1411, where the uncollapsed tuple conversion led to an assertion failing in the code generator