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Generation of type invariants VC (wip).

Somes experiments around the generation of type invariants implication.

metas are stored in a new field pd_metas aside the field pd_pure

- its_fragile for non-free mutable types must consider non-pure
  variables as liquid (= potentially mutable)

- non-free mutable types can have undetectable non-liable fragile
  mutable fields and in this case must be considered fragile.
  This is not ideal, because we will then reprove their proper
  invariant even though it is never at risk, but such types are not
  very plausible in the first place (indeed, we are talking about
  a mutable type with an invariant, stored in a mutable field of
  another type with an invariant, and this field is not mentioned
  in the outer invariant).

Treatment of ghost branchs