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Port of Interval Validation to mixed precision. Needed some auxiliary lemmas related to the typing of expressions.

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Merge some changes to ssa definitions and documentation

See merge request !84

Make certificate generation compliant with new format in upstream

See merge request !81

Finish proofs for new semantics in Coq

See merge request !80

Refactor absolute error computation (step 1)

See merge request !79

The following has changed:
- absolute error computation previously present in ErrorFunctions
  is now split into range computation and error computation.
- it does not annotate the tree anymore, the information is passed via maps in the context
- error computation supports mixed-precision (without a front-end)

What is probably broken or needs fixing:
- fixed-point code generation
- RangePhase and AbsErrorPhases cleanup and deduplication

Add a stable version of let Bindings to HOL4 and Coq certificates

See merge request !76