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- copy the code of Pretty to Why3 to prepare it for Driver
- move goal_of_ctxt to Transform, where it belongs 
- comment out the unused "extract_goals" in Transform
- comment out the debugging printing in Theory, use Pretty
- in use_export, put the Duse declaration after the copied
  declarations, not before

- prevent leak of idents by separating them into two types:
  - "preid" -- user-created and non-unique
  - "ident" -- unique, generated from "preid" by various
               smart constructors: create_tysymbol, etc

  This guarantees that two different symbols never share
  an ident.

- no need to hashcons tysymbols, fsymbols, and psymbols,
  as they are unique by construction

- make separate hashconsing smart constructors for decl

- export namespace as a private record (no reason to not to)

- some code rearrangement in hashconsing of decls

- fix namespace merging in close_namespace

- namespace name can be just a string, no need to use ident

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